Don't use --color if grep doesn't support it

Not all versions of grep support the --color flag. This checks the
output of grep --help when building the grep command and avoids using
flags that aren't compatible with the version present.

Fixes airblade#234.

autoload/gitgutter/diff.vim

@@ -1,5 +1,12 @@
 let s:grep_available = executable('grep')
-let s:grep_command = ' | '.(g:gitgutter_escape_grep ? '\grep' : 'grep').' --color=never -e '.gitgutter#utility#shellescape('^@@ ')
+if s:grep_available
+  let s:grep_command = ' | '.(g:gitgutter_escape_grep ? '\grep' : 'grep')
+  let s:grep_help = gitgutter#utility#system('grep --help')
+  if s:grep_help =~# '--color'
+    let s:grep_command .= ' --color=never'
+  endif
+  let s:grep_command .= ' -e '.gitgutter#utility#shellescape('^@@ ')
+endif
 let s:hunk_re = '^@@ -\(\d\+\),\?\(\d*\) +\(\d\+\),\?\(\d*\) @@'