Minimize use of combined stdout and stderr

Combined stdout and stderr is what system() gives us, so the plugin was
built around it.  But getting the same from jobs is annoying, so let's
eliminate all unnecessary uses of it.

autoload/fugitive.vim

@@ -557,6 +557,12 @@ function! s:ChompError(...) abort
   return [s:sub(out, '\n$', ''), exec_error]
 endfunction
 
+function! s:ChompStderr(...) abort
+  let [out, exec_error] = call('s:ChompError', a:000)
+  let out = substitute(out, "\n$", '', '')
+  return !exec_error ? '' ? len(out) : out : 'unknown Git error'
+endfunction
+
 function! s:ChompDefault(default, ...) abort
   let [out, exec_error] = call('s:ChompError', a:000)
   return exec_error ? a:default : out
@@ -592,7 +598,7 @@ function! s:EchoExec(...) abort
   if s:RunJobs()
     return 'Git ' . s:fnameescape(a:000)
   else
-    echo call('s:ChompError', a:000)[0]
+    echo substitute(s:SystemError(call('fugitive#Prepare', a:000))[0], "\n$", '', '')
     call fugitive#ReloadStatus(-1, 1)
     return 'checktime'
   endif
@@ -624,11 +630,11 @@ function! fugitive#Head(...) abort
 endfunction
 
 function! fugitive#RevParse(rev, ...) abort
-  let [hash, exec_error] = s:ChompError([a:0 ? a:1 : s:Dir(), 'rev-parse', '--verify', a:rev, '--'])
-  if !exec_error && hash =~# '^\x\{40,\}$'
+  let hash = s:ChompDefault('', [a:0 ? a:1 : s:Dir(), 'rev-parse', '--verify', a:rev, '--'])
+  if hash =~# '^\x\{40,\}$'
     return hash
   endif
-  throw 'fugitive: rev-parse '.a:rev.': '.hash
+  throw 'fugitive: failed to parse revision ' . a:rev
 endfunction
 
 function! s:ConfigTimestamps(dir, dict) abort
@@ -1511,8 +1517,8 @@ function! s:TreeInfo(dir, commit) abort
       let s:trees[a:dir] = {}
     endif
     if !has_key(s:trees[a:dir], a:commit)
-      let [ftime, exec_error] = s:ChompError([a:dir, 'log', '-1', '--pretty=format:%ct', a:commit, '--'])
-      if exec_error
+      let ftime = s:ChompDefault('', [a:dir, 'log', '-1', '--pretty=format:%ct', a:commit, '--'])
+      if empty(ftime)
         let s:trees[a:dir][a:commit] = [{}, -1]
         return s:trees[a:dir][a:commit]
       endif
@@ -1710,9 +1716,9 @@ function! fugitive#writefile(lines, url, ...) abort
       call writefile(fugitive#readfile(url, 'b'), temp, 'b')
     endif
     call call('writefile', [a:lines, temp] + a:000)
-    let [hash, exec_error] = s:ChompError([dir, 'hash-object', '-w', temp])
+    let hash = s:ChompDefault('', [dir, 'hash-object', '-w', temp])
     let mode = len(entry[1]) ? entry[1] : '100644'
-    if !exec_error && hash =~# '^\x\{40,\}$'
+    if hash =~# '^\x\{40,\}$'
       let error = s:UpdateIndex(dir, [mode, hash, commit, file[1:-1]])
       if empty(error)
         return 0
@@ -2385,7 +2391,7 @@ function! fugitive#FileWriteCmd(...) abort
     endif
     silent execute "noautocmd keepalt '[,']write ".temp
     let hash = s:TreeChomp([dir, 'hash-object', '-w', '--', temp])
-    let old_mode = matchstr(s:ChompError(['ls-files', '--stage', '.' . file], dir)[0], '^\d\+')
+    let old_mode = matchstr(s:ChompDefault('', ['ls-files', '--stage', '.' . file], dir), '^\d\+')
     if empty(old_mode)
       let old_mode = executable(s:Tree(dir) . file) ? '100755' : '100644'
     endif
@@ -4166,8 +4172,8 @@ function! s:StageApply(info, reverse, extra) abort
     call add(cmd, '--reverse')
   endif
   call extend(cmd, ['--', temp])
-  let [output, exec_error] = s:ChompError(cmd)
-  if !exec_error
+  let output = s:ChompStderr(cmd)
+  if empty(output)
     return 1
   endif
   call s:throw(output)
@@ -5355,13 +5361,9 @@ function! fugitive#WriteCommand(line1, line2, range, bang, mods, arg, args) abor
     execute 'write! '.s:fnameescape(file)
   endif
 
-  if a:bang
-    let [error, exec_error] = s:ChompError(['add', '--force', '--', file])
-  else
-    let [error, exec_error] = s:ChompError(['add', '--', file])
-  endif
-  if exec_error
-    let v:errmsg = 'fugitive: '.error
+  let message = s:ChompStderr(['add'] + (a:bang ? ['--force'] : []) + ['--', file])
+  if len(message)
+    let v:errmsg = 'fugitive: '.message
     return 'echoerr v:errmsg'
   endif
   if s:cpath(fugitive#Real(@%), file) && s:DirCommitFile(@%)[1] =~# '^\d$'
@@ -5711,8 +5713,8 @@ function! s:Move(force, rename, destination) abort
   if isdirectory(@%)
     setlocal noswapfile
   endif
-  let [message, exec_error] = s:ChompError(['mv'] + (a:force ? ['-f'] : []) + ['--', expand('%:p'), destination], dir)
-  if exec_error
+  let message = s:ChompStderr(['mv'] + (a:force ? ['-f'] : []) + ['--', expand('%:p'), destination], dir)
+  if len(message)
     let v:errmsg = 'fugitive: '.message
     return 'echoerr v:errmsg'
   endif
@@ -5761,8 +5763,8 @@ function! s:Remove(after, force) abort
   if a:force
     let cmd += ['--force']
   endif
-  let [message, exec_error] = s:ChompError(cmd + ['--', expand('%:p')], dir)
-  if exec_error
+  let message = s:ChompStderr(cmd + ['--', expand('%:p')], dir)
+  if len(message)
     let v:errmsg = 'fugitive: '.s:sub(message,'error:.*\zs\n\(.*-f.*',' (add ! to force)')
     return 'echoerr '.string(v:errmsg)
   else
@@ -5891,7 +5893,7 @@ function! s:BlameSubcommand(line1, count, range, bang, mods, options) abort
       let i += 1
       if i == len(flags)
         echohl ErrorMsg
-        echo s:ChompError([dir, 'blame', arg])[0]
+        echo s:ChompStderr([dir, 'blame', arg])
         echohl NONE
         return ''
       endif
@@ -6475,7 +6477,7 @@ function! fugitive#BrowseCommand(line1, count, range, bang, mods, arg, args) abo
       if r ==# '.' || r ==# remote
         let remote_ref = 'refs/remotes/' . remote . '/' . branch
         if FugitiveConfigGet('push.default', dir) ==# 'upstream' ||
-              \ !filereadable(FugitiveFind('.git/' . remote_ref, dir)) && s:ChompError(['rev-parse', '--verify', remote_ref, '--'], dir)[1]
+              \ !filereadable(FugitiveFind('.git/' . remote_ref, dir)) && empty(s:ChompDefault('', ['rev-parse', '--verify', remote_ref, '--'], dir))
           let merge = m
           if path =~# '^\.git/refs/heads/.'
             let path = '.git/refs/heads/'.merge
@@ -6495,10 +6497,7 @@ function! fugitive#BrowseCommand(line1, count, range, bang, mods, arg, args) abo
         let commit = ''
         if len(merge)
           let owner = s:Owner(@%, dir)
-          let [commit, exec_error] = s:ChompError(['merge-base', 'refs/remotes/' . remote . '/' . merge, empty(owner) ? '@' : owner, '--'], dir)
-          if exec_error
-            let commit = ''
-          endif
+          let commit = s:ChompDefault('', ['merge-base', 'refs/remotes/' . remote . '/' . merge, empty(owner) ? '@' : owner, '--'], dir)
           if line2 > 0 && empty(arg) && commit =~# '^\x\{40,\}$'
             let blame_list = tempname()
             call writefile([commit, ''], blame_list, 'b')