diff: fix --merge-base with annotated tags

diff: fix --merge-base with annotated tags

Checking early for OBJ_COMMIT excludes other objects that can be
resolved to commits, like annotated tags.  If we remove it, annotated
tags will be resolved and handled just fine by
lookup_commit_reference(), and if we are given something that can't be
resolved to a commit, we'll still get a useful error message, e.g.:

> error: object 21ab162211ac3ef13c37603ca88b27e9c7e0d40b is a tree, not a commit
> fatal: no merge base found

Signed-off-by: Alyssa Ross <hi@alyssa.is>
Signed-off-by: Junio C Hamano <gitster@pobox.com>