suggested feature: someone mails me a blob, git please tell me what it is

Here's a suggestion for a git feature, if git doesn't already have it.
 Since git generally doesn't use RCS-style keyword cookies (like
"$Id$"), it'd be nice to have some other way to identify an installed
file.  Perhaps you'd want to know exactly what rev a user is running,
for example.

It seems like it should be fairly easy, given a blob (the file), for git to describe what it knows about it. For example, it could provide a list of commits that it's a part of, etc. It'd be *really* nice if only the output of the sha1sum command on the blob were needed. (I can't recall exactly how git's blob SHA1's are computed--maybe this isn't feasible.)

Mike