When we allow a tag object in place of a commit object, we only
dereferenced the given tag once, which causes a tag that points at a tag
that points at a commit to be rejected.  Instead, dereference tag
repeatedly until we get a non-tag.

This patch makes change to two functions:

 - commit.c::lookup_commit_reference() is used by merge-base,
   rev-tree and rev-parse to convert user supplied SHA1 to that of
   a commit.
 - rev-list uses its own get_commit_reference() to do the same.

Dereferencing tags this way helps both of these uses.
Signed-off-by: NJunio C Hamano <junkio@cox.net>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>