Check for grep

Fixes #72

README.mkd

@@ -218,6 +218,7 @@ Here are some things you can check:
 * Your git config is compatible with the version of git which your Vim is calling (`:echo system('git --version')`).
 * Your Vim supports signs (`:echo has('signs')` should give `1`).
 * Your file is being tracked by git and has unstaged changes.
+* `grep` is on your path and available to Vim (`:echo executable('grep')` should return 1)
 
 
 ### Alternatives

plugin/gitgutter.vim

@@ -1,4 +1,4 @@
-if exists('g:loaded_gitgutter') || !executable('git') || !has('signs') || &cp
+if exists('g:loaded_gitgutter') || !executable('git') || !executable('grep') || !has('signs') || &cp
   finish
 endif
 let g:loaded_gitgutter = 1