We had originally copied this function in at a time when notmuch
wasn't actually depending on the GMime library. Now that it does,
we might as well call the function that exists there rather
than having a private copy of it.
This way both the .c and .h files have the same name, and all of the
code imported from the "libsha1" implementation is in filenames
matching libsha1.*.
This also gives me room to make my own notmuch_sha1 wrapper functions
in sha1.c.
Keith's name already shows up in the git log, so it would be
wrong to not mention him. And Martin and Jamey have been
helpful in discussions about what an ideal mail system
would look like.
