Stop early when file is in .git directory.

autoload/gitgutter/utility.vim

@@ -9,8 +9,14 @@ function! gitgutter#utility#warn(message)
   let v:warningmsg = a:message
 endfunction
 
+" Returns truthy when the buffer's file should be processed; and falsey when it shouldn't.
+" This function does not and should not make any system calls.
 function! gitgutter#utility#is_active()
-  return g:gitgutter_enabled && gitgutter#utility#exists_file()
+  return g:gitgutter_enabled && gitgutter#utility#exists_file() && gitgutter#utility#not_git_dir()
+endfunction
+
+function! gitgutter#utility#not_git_dir()
+  return gitgutter#utility#full_path_to_directory_of_file() !~ '\.git\([/\\].*\)\?$'
 endfunction
 
 " A replacement for the built-in `shellescape(arg)`.
@@ -51,6 +57,10 @@ function! gitgutter#utility#extension()
   return fnamemodify(s:file, ':e')
 endfunction
 
+function! gitgutter#utility#full_path_to_directory_of_file()
+  return fnamemodify(s:file, ':p:h')
+endfunction
+
 function! gitgutter#utility#directory_of_file()
   return fnamemodify(s:file, ':h')
 endfunction