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>

ui-shared.c

@@ -1133,6 +1133,13 @@ void cgit_print_snapshot_links(const struct cgit_repo *repo, const char *ref,
 		strbuf_addstr(&filename, f->suffix);
 		cgit_snapshot_link(filename.buf, NULL, NULL, NULL, NULL,
 				   filename.buf);
+		if (cgit_snapshot_get_sig(ref, f)) {
+			strbuf_addstr(&filename, ".asc");
+			html(" (");
+			cgit_snapshot_link("sig", NULL, NULL, NULL, NULL,
+					   filename.buf);
+			html(")");
+		}
 		html(separator);
 	}
 	strbuf_release(&filename);