"Files" matches the find string against each of the files modified
by each commit, and can do exact, case-ignoring or regexp matching.

"Pickaxe" uses git-diff-tree -S'string' and can only do exact
matching.  I called it "pickaxe" rather than "find within patch"
since it only finds commits where the string is present in the child
but not the parents or vice versa, and "pickaxe" is what the author
of that feature calls it.