Move git-tracking check into diff command.

This avoids shelling out twice per buffer: once to check whether git
knows about the file and once to perform the diff.  Now we simply do
both in one external call.

Profiling showed external calls to git taking ~20ms.  This doesn't seem
too bad but it adds up.

autoload/diff.vim

@@ -4,17 +4,25 @@ let s:hunk_re = '^@@ -\(\d\+\),\?\(\d*\) +\(\d\+\),\?\(\d*\) @@'
 
 
 function! diff#run_diff(realtime, use_external_grep)
+  let cmd = 'git ls-files --error-unmatch' . utility#discard_stdout_and_stderr() . ' ' . shellescape(utility#file()) . ' && '
+
   if a:realtime
     let blob_name = ':./' . fnamemodify(utility#file(),':t')
     let blob_file = tempname()
-    let cmd = 'git show ' . blob_name . ' > ' . blob_file . ' && diff -U0 ' . g:gitgutter_diff_args . ' ' . blob_file . ' - '
+    let cmd .= 'git show ' . blob_name . ' > ' . blob_file .
+          \ ' && diff -U0 ' . g:gitgutter_diff_args . ' ' . blob_file . ' - '
   else
-    let cmd = 'git diff --no-ext-diff --no-color -U0 ' . g:gitgutter_diff_args . ' ' . shellescape(utility#file())
+    let cmd .= 'git diff --no-ext-diff --no-color -U0 ' . g:gitgutter_diff_args . ' ' . shellescape(utility#file())
   endif
+
   if a:use_external_grep && s:grep_available
-    let cmd .= s:grep_command
+    " grep exits with 1 when no matches are found.  However we want to treat
+    " non-matches as non-erroneous behaviour; so we append ` || :`.
+    let cmd .= s:grep_command . ' || :'
   endif
+
   let cmd = utility#escape(cmd)
+
   if a:realtime
     if &fileformat ==# "dos"
       let eol = "\r\n"
@@ -28,6 +36,13 @@ function! diff#run_diff(realtime, use_external_grep)
   else
     let diff = system(utility#command_in_directory_of_file(cmd))
   endif
+
+  if v:shell_error
+    " A shell error indicates the file is not tracked by git (unless something
+    " bizarre is going on).
+    throw 'git-diff failed'
+  endif
+
   return diff
 endfunction
 

autoload/utility.vim

@@ -1,7 +1,7 @@
 let s:file = ''
 
 function! utility#is_active()
-  return g:gitgutter_enabled && utility#exists_file() && utility#is_tracked_by_git()
+  return g:gitgutter_enabled && utility#exists_file()
 endfunction
 
 function! utility#current_file()
@@ -64,12 +64,6 @@ function! utility#command_in_directory_of_file(cmd)
   return substitute(utility#cmd_in_dir, "'", '"', 'g')
 endfunction
 
-function! utility#is_tracked_by_git()
-  let cmd = utility#escape('git ls-files --error-unmatch' . utility#discard_stdout_and_stderr() . ' ' . shellescape(utility#file()))
-  call system(utility#command_in_directory_of_file(cmd))
-  return !v:shell_error
-endfunction
-
 function! utility#differences(hunks)
   return len(a:hunks) != 0
 endfunction

plugin/gitgutter.vim

@@ -55,26 +55,30 @@ command GitGutterAll call GitGutterAll()
 function! GitGutter(file, realtime)
   call utility#set_file(a:file)
   if utility#is_active()
-    if !a:realtime || utility#has_fresh_changes(a:file)
+    try
-      let diff           = diff#run_diff(a:realtime || utility#has_unsaved_changes(a:file), 1)
+      if !a:realtime || utility#has_fresh_changes(a:file)
-      let s:hunks        = diff#parse_diff(diff)
+        let diff           = diff#run_diff(a:realtime || utility#has_unsaved_changes(a:file), 1)
-      let modified_lines = diff#process_hunks(s:hunks)
+        let s:hunks        = diff#parse_diff(diff)
+        let modified_lines = diff#process_hunks(s:hunks)
 
-      if g:gitgutter_signs
+        if g:gitgutter_signs
-        if g:gitgutter_sign_column_always
+          if g:gitgutter_sign_column_always
-          call sign#add_dummy_sign()
+            call sign#add_dummy_sign()
-        else
-          if utility#differences(s:hunks)
-            call sign#add_dummy_sign()  " prevent flicker
           else
-            call sign#remove_dummy_sign()
+            if utility#differences(s:hunks)
+              call sign#add_dummy_sign()  " prevent flicker
+            else
+              call sign#remove_dummy_sign()
+            endif
           endif
+          call sign#update_signs(a:file, modified_lines)
         endif
-        call sign#update_signs(a:file, modified_lines)
-      endif
 
-      call utility#save_last_seen_change(a:file)
+        call utility#save_last_seen_change(a:file)
-    endif
+      endif
+    catch /git-diff failed/
+      call hunk#reset()
+    endtry
   else
     call hunk#reset()
   endif