Fix crash in pre-receive hook when pushing annotated tag

The value of an annotated tag is the SHA-1 of a tag object, not a commit
object.  Use "git rev-parse $sha1^{commit}" to resolve the SHA1s given to
index.processCommits() before using them as direct commit references.

Recent regression introduced by:

6ef0eed5 "Common up calls to index.processCommits()"