push: add reflog check for "--force-if-includes"

push: add reflog check for "--force-if-includes"

Add a check to verify if the remote-tracking ref of the local branch
is reachable from one of its "reflog" entries.

The check iterates through the local ref's reflog to see if there
is an entry for the remote-tracking ref and collecting any commits
that are seen, into a list; the iteration stops if an entry in the
reflog matches the remote ref or if the entry timestamp is older
the latest entry of the remote ref's "reflog". If there wasn't an
entry found for the remote ref, "in_merge_bases_many()" is called
to check if it is reachable from the list of collected commits.

When a local branch that is based on a remote ref, has been rewound
and is to be force pushed on the remote, "--force-if-includes" runs
a check that ensures any updates to the remote-tracking ref that may
have happened (by push from another repository) in-between the time
of the last update to the local branch (via "git-pull", for instance)
and right before the time of push, have been integrated locally
before allowing a forced update.

If the new option is passed without specifying "--force-with-lease",
or specified along with "--force-with-lease=<refname>:<expect>" it
is a "no-op".

Signed-off-by: Srinidhi Kaushik <shrinidhi.kaushik@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>