Always use bash on unix.

Vim is single-threaded so we can make life easier by using a known shell
and restoring the original one afterwards.

autoload/gitgutter.vim

@@ -14,6 +14,8 @@ endfunction
 " bufnr: (integer) the buffer to process.
 " realtime: (boolean) when truthy, do a realtime diff; otherwise do a disk-based diff.
 function! gitgutter#process_buffer(bufnr, realtime) abort
+  call gitgutter#utility#use_known_shell()
+
   call gitgutter#utility#set_buffer(a:bufnr)
   if gitgutter#utility#is_active()
     if g:gitgutter_sign_column_always
@@ -34,6 +36,8 @@ function! gitgutter#process_buffer(bufnr, realtime) abort
   else
     call gitgutter#hunk#reset()
   endif
+
+  call gitgutter#utility#restore_shell()
 endfunction
 
 

autoload/gitgutter/diff.vim

@@ -9,8 +9,6 @@ else
 endif
 let s:hunk_re = '^@@ -\(\d\+\),\?\(\d*\) +\(\d\+\),\?\(\d*\) @@'
 
-let s:fish = &shell =~# 'fish'
-
 let s:c_flag = gitgutter#utility#git_supports_command_line_config_override()
 
 let s:temp_index = tempname()
@@ -54,14 +52,13 @@ let s:temp_buffer = tempname()
 " does the filtering instead.
 function! gitgutter#diff#run_diff(realtime, preserve_full_diff) abort
   " Wrap compound commands in parentheses to make Windows happy.
-  " bash doesn't mind the parentheses; fish doesn't want them.
-  let cmd = s:fish ? '' : '('
+  " bash doesn't mind the parentheses.
+  let cmd = '('
 
   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 .= s:fish ? '; and ' : ' && ('
+    let cmd .= 'git ls-files --error-unmatch '.gitgutter#utility#shellescape(gitgutter#utility#filename()).' && ('
   endif
 
   if a:realtime
@@ -73,8 +70,7 @@ function! gitgutter#diff#run_diff(realtime, preserve_full_diff) abort
       let blob_file .= '.'.extension
       let buff_file .= '.'.extension
     endif
-    let cmd .= 'git show '.blob_name.' > '.blob_file
-    let cmd .= s:fish ? '; and ' : ' && '
+    let cmd .= 'git show '.blob_name.' > '.blob_file.' && '
 
     " Writing the whole buffer resets the '[ and '] marks and also the
     " 'modified' flag (if &cpoptions includes '+').  These are unwanted
@@ -111,17 +107,14 @@ function! gitgutter#diff#run_diff(realtime, preserve_full_diff) abort
     " 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 .= s:fish ? '; or ' : ' || '
-    let cmd .= 'exit 0'
+    let cmd .= ' || exit 0'
   endif
 
-  if !s:fish
   let cmd .= ')'
 
   if !tracked
     let cmd .= ')'
   endif
-  end
 
   let cmd = gitgutter#utility#command_in_directory_of_file(cmd)
 

autoload/gitgutter/utility.vim

@@ -1,7 +1,6 @@
 let s:file = ''
 let s:using_xolox_shell = -1
 let s:exit_code = 0
-let s:fish = &shell =~# 'fish'
 
 function! gitgutter#utility#warn(message) abort
   echohl WarningMsg
@@ -148,7 +147,7 @@ function! gitgutter#utility#file_relative_to_repo_root() abort
 endfunction
 
 function! gitgutter#utility#command_in_directory_of_file(cmd) abort
-  return 'cd '.gitgutter#utility#shellescape(gitgutter#utility#directory_of_file()) . (s:fish ? '; and ' : ' && ') . a:cmd
+  return 'cd '.gitgutter#utility#shellescape(gitgutter#utility#directory_of_file()).' && '.a:cmd
 endfunction
 
 function! gitgutter#utility#highlight_name_for_change(text) abort
@@ -182,3 +181,19 @@ endfunction
 function! gitgutter#utility#stringify(list) abort
   return join(a:list, "\n")."\n"
 endfunction
+
+function! gitgutter#utility#use_known_shell() abort
+  if has('unix')
+    let s:shell = &shell
+    let s:shellcmdflag = &shellcmdflag
+    set shell=/bin/bash
+    set shellcmdflag=-c
+  endif
+endfunction
+
+function! gitgutter#utility#restore_shell() abort
+  if has('unix')
+    let &shell = s:shell
+    let &shellcmdflag = s:shellcmdflag
+  endif
+endfunction