Git ships with a command called grep that allows you to easily search through any committed tree or the working directory for a string or regular expression.
Also, git log command has a number of powerful tools for finding specific commits by the content of their messages or even the content of the diff they introduce.
Search in all of the commit messages of project
git log --grep="foo"
Search in commit content (in diffs)
git grep "foo" $(git rev-list --all)
or using 'pickaxe' feature of git log grep,
git log -S"foo"
Using 'pickaxe' feature and show commit content
git log -p -S"foo"
Using 'pickaxe' feature and search over all branches
git log --all -S"foo"
alternatively, use --branches[=<pattern>] or --tags[=<pattern>].
Search working tree for text matching regular expression regexp
git grep "regexp"
Search working tree for lines of text matching regular expression regexp1 or regexp2
git grep -e regexp1 --or -e regexp2
Search working tree for lines of text matching regular expression regexp1 and regexp2, reporting file paths only:
git grep -e regexp1 --and -e regexp2