Add new git-rm command with documentation

Add new git-rm command with documentation

This adds a git-rm command which provides convenience similar to
git-add, (and a bit more since it takes care of the rm as well if
given -f).

Like git-add, git-rm expands the given path names through
git-ls-files. This means it only acts on files listed in the
index. And it does act recursively on directories by default, (no -r
needed as in the case of rm itself). When it recurses, it does not
remove empty directories that are left behind.

Signed-off-by: Junio C Hamano <junkio@cox.net>