Search all of Git history for a string?

git rev-list --all | (
    while read revision; do
        git grep -F 'password' $revision
    done
)

Git can search diffs with the -S option (it's called pickaxe in the docs)

git log -S password

This will find any commit that added or removed the string password. Here a few options:

• -p: will show the diffs. If you provide a file (-p file), it will generate a patch for you.
• -G: looks for differences whose added or removed line matches the given regexp, as opposed to -S, which "looks for differences that introduce or remove an instance of string".
• --all: searches over all branches and tags; alternatively, use --branches[=<pattern>] or --tags[=<pattern>]