When coding, I often need to search for the references of a variable or a function across multiple files. Usually I enter command mode of vim and do a grep command, memorize the search results and open the files having hits. This common pattern could be dramatically accelerated by a user-defined vim command:
:command Gitgrep :lex system("git grep -n " . shellescape(getreg('/'))).

4b825dc642cb6eb9a060e54bf8d69288fbee4904 is the hash code to index the “empty tree” (before your first commit) in git version control system.

This post talks about how to get patch work with git.