Normally version control systems don't allow fine grained commits.
commit-patch allows the user to control exactly what gets committed by
letting the user supply a patch to be committed rather than using the
files in the current working directory.

commit-patch supports Darcs, Git, Mercurial, Bazaar, Subversion,
Monotone or CVS repositories.

Also included is an Emacs interface to commit-patch. It allows you to
just hit C-c C-c in any patch buffer to apply and commit only the
changes indicated by the patch, regardless of the changes in your
working directory.