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>

cgit.h

@@ -88,6 +88,7 @@ struct cgit_repo {
 	char *clone_url;
 	char *logo;
 	char *logo_link;
+	char *snapshot_prefix;
 	int snapshots;
 	int enable_commit_graph;
 	int enable_log_filecount;