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-shared.h

@@ -78,6 +78,7 @@ extern void cgit_compose_snapshot_prefix(struct strbuf *filename,
 					 const char *base, const char *ref);
 extern void cgit_print_snapshot_links(const struct cgit_repo *repo,
 				      const char *head, const char *hex);
+extern const char *cgit_snapshot_prefix(const struct cgit_repo *repo);
 extern void cgit_add_hidden_formfields(int incl_head, int incl_search,
 				       const char *page);