Minor refactor.

autoload/gitgutter/diff.vim

@@ -9,23 +9,18 @@ if s:grep_available
 endif
 let s:hunk_re = '^@@ -\(\d\+\),\?\(\d*\) +\(\d\+\),\?\(\d*\) @@'
 
+let s:fish = &shell =~# 'fish'
 
 function! gitgutter#diff#run_diff(realtime, use_external_grep)
-  " Detect whether user uses fish shell
-  let l:fish = &shell =~# 'fish'
-
-  if !l:fish
-    " Wrap compound commands in parentheses to make Windows happy.
-    let cmd = '('
-  else
-    let cmd = ''
-  end
+  " Wrap compound commands in parentheses to make Windows happy.
+  " bash doesn't mind the parentheses; fish doesn't want them.
+  let cmd = s:fish ? '' : '('
 
   let bufnr = gitgutter#utility#bufnr()
   let tracked = getbufvar(bufnr, 'gitgutter_tracked')  " i.e. tracked by git
   if !tracked
     let cmd .= 'git ls-files --error-unmatch '.gitgutter#utility#shellescape(gitgutter#utility#filename())
-    let cmd .= l:fish ? '; and ' : ' && ('
+    let cmd .= s:fish ? '; and ' : ' && ('
   endif
 
   if a:realtime
@@ -38,7 +33,7 @@ function! gitgutter#diff#run_diff(realtime, use_external_grep)
       let buff_file .= '.'.extension
     endif
     let cmd .= 'git show '.blob_name.' > '.blob_file
-    let cmd .= l:fish ? '; and ' : ' && '
+    let cmd .= s:fish ? '; and ' : ' && '
 
     " Writing the whole buffer resets the '[ and '] marks and also the
     " 'modified' flag (if &cpoptions includes '+').  These are unwanted
@@ -70,10 +65,11 @@ function! gitgutter#diff#run_diff(realtime, use_external_grep)
     " differences are found.  However we want to treat non-matches and
     " differences as non-erroneous behaviour; so we OR the command with one
     " which always exits with success (0).
-    let cmd.= l:fish ? '; or exit 0' : ' || exit 0'
+    let cmd .= s:fish ? '; or ' : ' || '
+    let cmd .= 'exit 0'
   endif
 
-  if !l:fish
+  if !s:fish
     let cmd .= ')'
 
     if !tracked