Make robust the replacement of temp file paths with actual paths.

The previous implementation meant the temp file paths were treated as
regular expressions, which was vulnerable to problems with backslashes
etc.

See #494.

autoload/gitgutter/hunk.vim

@@ -228,13 +228,28 @@ endfunction
 
 " Replaces references to temp files with the actual file.
 function! s:fix_file_references(filepath, hunk_diff)
-  let diff = a:hunk_diff
-  for tmp in matchlist(diff, '\vdiff --git ./(\S+) ./(\S+)\n')[1:2]
-    let diff = substitute(diff, tmp, a:filepath, 'g')
-  endfor
-  return diff
+  let lines = split(a:hunk_diff, '\n')
+
+  let left_prefix  = matchstr(lines[2], '[abciow12]').'/'
+  let right_prefix = matchstr(lines[3], '[abciow12]').'/'
+  let quote        = lines[0][11] == '"' ? '"' : ''
+
+  let left_file  = quote.left_prefix.a:filepath.quote
+  let right_file = quote.right_prefix.a:filepath.quote
+
+  let lines[0] = 'diff --git '.left_file.' '.right_file
+  let lines[2] = '--- '.left_file
+  let lines[3] = '+++ '.right_file
+
+  return join(lines, "\n")."\n"
 endfunction
 
+if $TEST
+  function! gitgutter#hunk#fix_file_references(filepath, hunk_diff)
+    return s:fix_file_references(a:filepath, a:hunk_diff)
+  endfunction
+endif
+
 
 function! s:adjust_hunk_summary(hunk_diff) abort
   let line_adjustment = s:line_adjustment_for_current_hunk()