Everybody envies rev-parse, who is the only one that can grok
the extended sha1 format. Move the get_extended_sha1() out of
rev-parse, rename it to get_sha1() and make it available to
everybody else.
The one I posted earlier to the list had one bug where it did
not handle a name that ends with a digit correctly (it
incorrectly tried the "Nth parent" path). This commit fixes it.