Use /bin/sh as default shell.

- /bin/bash is not part of OpenBSD.
- We need only a POSIX shell, not bash specifically.

Closes #340.

autoload/gitgutter/utility.vim

@@ -186,7 +186,7 @@ function! gitgutter#utility#use_known_shell() abort
   if has('unix')
     let s:shell = &shell
     let s:shellcmdflag = &shellcmdflag
-    set shell=/bin/bash
+    set shell=/bin/sh
     set shellcmdflag=-c
   endif
 endfunction