snapshot: support archive signatures

snapshot: support archive signatures

Read signatures from the notes refs refs/notes/signatures/$FORMAT where
FORMAT is one of our archive formats ("tar", "tar.gz", ...).  The note
is expected to simply contain the signature content to be returned when
the snapshot "${filename}.asc" is requested, so the signature for
cgit-1.1.tar.xz can be stored against the v1.1 tag with:

	git notes --ref=refs/notes/signatures/tar.xz add -C "$(
		gpg --output - --armor --detach-sign cgit-1.1.tar.xz |
		git hash-object -w --stdin
	)" v1.1

and then downloaded by simply appending ".asc" to the archive URL.

Signed-off-by: John Keeping <john@keeping.me.uk>
Reviewed-by: Christian Hesse <mail@eworm.de>