[Update 2010-09-20: tweaked to run the grep on the git root instead of whatever directory the current file is in.]

After many months of screwing around with git.vim and fugitive.vim, I have finally found the perfect vim + git grep combination.

This incantation allows you to press <ctrl-x> twice on a symbol and have a minibuf pop up with all the occurrences of that symbol within the project. You can then jump to any occurrence by pressing enter on the corresponding line.

Place this in e.g. ~/.vim/plugin/git-grep.vim:

let g:gitgrepprg="git\\ grep\\ -n"
let g:gitroot="`git rev-parse --show-cdup`"

function! GitGrep(args)
    let grepprg_bak=&grepprg
    exec "set grepprg=" . g:gitgrepprg
    execute "silent! grep " . a:args . " " . g:gitroot
    botright copen
    let &grepprg=grepprg_bak
    exec "redraw!"
endfunction

func GitGrepWord()
  normal! "zyiw
  call GitGrep(getreg('z'))
endf
nmap <C-x><C-x> :call GitGrepWord()<CR>

The vim ruby syntax seems to screw up comments that have multiple hashes. E.g. I like to differentiate

### section heading comments,
## non-inline comments, and
x = a + b # inline comments

But reformatting the comments (e.g. with “gq}”) always screws them up, unless you do:

$ mkdir -p ~/.vim/after/syntax
$ cat > ~/.vim/after/syntax/ruby.vim
set comments=n:#

which tells vim that multiple hash marks are ok.