Return empty string on FugitiveFind() with no Git dir

Previously, we would return a path from the current working directory in
this case, which was a good fallback for :Gedit but unhelpful for
general programmatic usage.

autoload/fugitive.vim

@@ -914,7 +914,7 @@ function! s:Owner(path, ...) abort
     elseif commit ==# '0'
       return ''
     endif
-    let merge_head = s:MergeHead()
+    let merge_head = s:MergeHead(dir)
     if empty(merge_head)
       return ''
     endif
@@ -1023,10 +1023,10 @@ function! fugitive#Find(object, ...) abort
   endif
   let dir = a:0 ? a:1 : s:Dir()
   if empty(dir)
-    let file = matchstr(a:object, '^\%(:\d:\|[^:]*:\)\zs.*', '', '')
+    let file = matchstr(a:object, '^\%(:\d:\|[^:]*:\)\zs\%(\.\.\=$\|\.\.\=/.*\|/.*\|\w:/.*\)')
     let dir = FugitiveExtractGitDir(file)
     if empty(dir)
-      return fnamemodify(FugitiveVimPath(len(file) ? file : a:object), ':p')
+      return ''
     endif
   endif
   let tree = s:Tree(dir)
@@ -1049,7 +1049,7 @@ function! fugitive#Find(object, ...) abort
       let f = simplify(dir . f)
     endif
   elseif rev ==# ':/'
-    let f = base
+    let f = tree
   elseif rev =~# '^\.\%(/\|$\)'
     let f = base . rev[1:-1]
   elseif rev =~# '^::\%(/\|\a\+\:\)'
@@ -1080,6 +1080,8 @@ function! fugitive#Find(object, ...) abort
     let f = matchstr(rev, ')\zs.*')
     if f=~# '^\.\.\=\%(/\|$\)'
       let f = simplify(getcwd() . '/' . f)
+    elseif empty(f)
+      let f = base
     elseif f !~# '^/\|^\%(\a\a\+:\).*\%(//\|::\)' . (has('win32') ? '\|^\a:/' : '')
       let f = base . '/' . f
     endif
@@ -1106,7 +1108,7 @@ function! fugitive#Find(object, ...) abort
         call map(commits, 'empty(v:val) || v:val ==# "@" ? "HEAD" : v:val')
         let commit = matchstr(s:ChompDefault('', [dir, 'merge-base'] + commits + ['--']), '\<[0-9a-f]\{40,\}\>')
       endif
-      if commit !~# '^[0-9a-f]\{40,\}$'
+      if commit !~# '^[0-9a-f]\{40,\}$\|^$'
         let commit = matchstr(s:ChompDefault('', [dir, 'rev-parse', '--verify', commit . (len(file) ? '^{}' : ''), '--']), '\<[0-9a-f]\{40,\}\>')
         if empty(commit) && len(file)
           let commit = repeat('0', 40)
@@ -1122,8 +1124,16 @@ function! fugitive#Find(object, ...) abort
   return FugitiveVimPath(f)
 endfunction
 
-function! s:Generate(rev, ...) abort
+function! s:Generate(object, ...) abort
-  return fugitive#Find(a:rev, a:0 ? a:1 : s:Dir())
+  let dir = a:0 ? a:1 : s:Dir()
+  let f = fugitive#Find(a:object, dir)
+  if !empty(f)
+    return f
+  elseif a:object ==# ':/'
+    return len(dir) ? FugitiveVimPath('fugitive://' . dir . '//0') : '.'
+  endif
+  let file = matchstr(a:object, '^\%(:\d:\|[^:]*:\)\zs.*')
+  return fnamemodify(FugitiveVimPath(len(file) ? file : a:object), ':p')
 endfunction
 
 function! s:DotRelative(path, ...) abort
@@ -4890,7 +4900,7 @@ function! s:OpenParse(string, wants_cmd) abort
   endif
   let dir = s:Dir()
   let efile = s:Expand(file)
-  let url = fugitive#Find(efile, dir)
+  let url = s:Generate(efile, dir)
 
   if a:wants_cmd && file[0] ==# '>' && efile[0] !=# '>' && get(b:, 'fugitive_type', '') isnot# 'tree' && &filetype !=# 'netrw'
     let line = line('.')
@@ -5133,9 +5143,9 @@ function! fugitive#WriteCommand(line1, line2, range, bang, mods, arg, args) abor
     setlocal nomodified
   endif
 
-  let one = s:Generate(':1:'.file)
+  let one = fugitive#Find(':1:'.file)
-  let two = s:Generate(':2:'.file)
+  let two = fugitive#Find(':2:'.file)
-  let three = s:Generate(':3:'.file)
+  let three = fugitive#Find(':3:'.file)
   for nr in range(1,bufnr('$'))
     let name = fnamemodify(bufname(nr), ':p')
     if bufloaded(nr) && !getbufvar(nr,'&modified') && (name ==# one || name ==# two || name ==# three)
@@ -5144,7 +5154,7 @@ function! fugitive#WriteCommand(line1, line2, range, bang, mods, arg, args) abor
   endfor
 
   unlet! restorewinnr
-  let zero = s:Generate(':0:'.file)
+  let zero = fugitive#Find(':0:'.file)
   silent exe s:DoAutocmd('BufWritePost ' . s:fnameescape(zero))
   for tab in range(1,tabpagenr('$'))
     for winnr in range(1,tabpagewinnr(tab,'$'))
@@ -5356,7 +5366,7 @@ function! fugitive#Diffsplit(autodir, keepfocus, mods, arg, args) abort
       exe pre
       let mods = (a:autodir ? s:diff_modifier(len(parents) + 1) : '') . s:Mods(mods, 'leftabove')
       let nr = bufnr('')
-      execute mods 'split' s:fnameescape(s:Generate(parents[0]))
+      execute mods 'split' s:fnameescape(fugitive#Find(parents[0]))
       call s:Map('n', 'dp', ':diffput '.nr.'<Bar>diffupdate<CR>', '<silent>')
       let nr2 = bufnr('')
       call s:diffthis()
@@ -5364,7 +5374,7 @@ function! fugitive#Diffsplit(autodir, keepfocus, mods, arg, args) abort
       call s:Map('n', 'd2o', ':diffget '.nr2.'<Bar>diffupdate<CR>', '<silent>')
       let mods = substitute(mods, '\Cleftabove\|rightbelow\|aboveleft\|belowright', '\=submatch(0) =~# "f" ? "rightbelow" : "leftabove"', '')
       for i in range(len(parents)-1, 1, -1)
-        execute mods 'split' s:fnameescape(s:Generate(parents[i]))
+        execute mods 'split' s:fnameescape(fugitive#Find(parents[i]))
         call s:Map('n', 'dp', ':diffput '.nr.'<Bar>diffupdate<CR>', '<silent>')
         let nrx = bufnr('')
         call s:diffthis()
@@ -6561,7 +6571,7 @@ function! s:StatusCfile(...) abort
 endfunction
 
 function! fugitive#StatusCfile() abort
-  let file = s:Generate(s:StatusCfile()[0])
+  let file = fugitive#Find(s:StatusCfile()[0])
   return empty(file) ? fugitive#Cfile() : s:fnameescape(file)
 endfunction
 
@@ -6591,7 +6601,7 @@ function! s:MessageCfile(...) abort
 endfunction
 
 function! fugitive#MessageCfile() abort
-  let file = s:Generate(s:MessageCfile())
+  let file = fugitive#Find(s:MessageCfile())
   return empty(file) ? fugitive#Cfile() : s:fnameescape(file)
 endfunction