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.c

@@ -79,6 +79,8 @@ static void repo_config(struct cgit_repo *repo, const char *name, const char *va
 		item->util = xstrdup(value);
 	} else if (!strcmp(name, "section"))
 		repo->section = xstrdup(value);
+	else if (!strcmp(name, "snapshot-prefix"))
+		repo->snapshot_prefix = xstrdup(value);
 	else if (!strcmp(name, "readme") && value != NULL) {
 		if (repo->readme.items == ctx.cfg.readme.items)
 			memset(&repo->readme, 0, sizeof(repo->readme));