Add "snapshot-prefix" repo configuration

Allow using a user-specified value for the prefix in snapshot files
instead of the repository basename.  For example, files downloaded from
the linux-stable.git repository should be named linux-$VERSION and not
linux-stable-$VERSION, which can be achieved by setting:

	repo.snapshot-prefix=linux

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

ui-refs.c

@@ -100,7 +100,7 @@ static void print_tag_downloads(const struct cgit_repo *repo, const char *ref)
 	if (!ref || strlen(ref) < 1)
 		return;
 
-	basename = cgit_repobasename(repo->url);
+	basename = cgit_snapshot_prefix(repo);
 	if (starts_with(ref, basename))
 		strbuf_addstr(&filename, ref);
 	else