ui-snapshot.c: specify archiver_args.baselen

The struct member was introduces in git commit d53fe8187c38, but the cgit
testsuite failed to detect that cgit always generated archives without
prefixes, i.e. the result from cgit_repobasename was ignored.

This fixes the bug and the testsuite.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>
diff --git a/ui-snapshot.c b/ui-snapshot.c
index 966a140..7a597ff 100644
--- a/ui-snapshot.c
+++ b/ui-snapshot.c
@@ -82,7 +82,13 @@
 		return 1;
 	}
 	memset(&args, 0, sizeof(args));
-	args.base = fmt("%s/", prefix);
+	if (prefix) {
+		args.base = fmt("%s/", prefix);
+		args.baselen = strlen(prefix) + 1;
+	} else {
+		args.base = "";
+		args.baselen = 0;
+	}
 	args.tree = commit->tree;
 	args.time = commit->date;
 	ctx.page.mimetype = xstrdup(format->mimetype);