multimail: simplify GitObject by replacing .commit

The commit attribute of a GitObject object serves the sole purpose of
conveying the SHA-1 of the underlying commit if the object is a tag.
Replace it with a commit_sha1 member to reduce the overhead.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Merge branch 'simplify-gitobject'

Replace member GitObject.commit with lighter-weight
GitObject.commit_sha1.