rebase: generify reset_head()

rebase: generify reset_head()

In the future, we plan on lib-ifying reset_head() so we need it to
be more generic. Make it more generic by making it accept a
`struct repository` argument instead of implicitly using the non-repo
functions. Also, make it accept a `const char *default_reflog_action`
argument so that the default action of "rebase" isn't hardcoded in.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>