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>
diff --git a/ui-snapshot.c b/ui-snapshot.c
index 237a75f..b9e2a36 100644
--- a/ui-snapshot.c
+++ b/ui-snapshot.c
@@ -154,7 +154,7 @@
 	if (get_oid(snapshot.buf, &oid) == 0)
 		goto out;
 
-	reponame = cgit_repobasename(repo->url);
+	reponame = cgit_snapshot_prefix(repo);
 	if (starts_with(snapshot.buf, reponame)) {
 		const char *new_start = snapshot.buf;
 		new_start += strlen(reponame);
@@ -214,7 +214,7 @@
 		hex = head;
 
 	if (!prefix)
-		prefix = xstrdup(cgit_repobasename(ctx.repo->url));
+		prefix = xstrdup(cgit_snapshot_prefix(ctx.repo));
 
 	make_snapshot(f, hex, prefix, filename);
 	free(prefix);