ui-snapshot.c: Prepend "V" when guessing ref names

In cgit_print_snapshot_links() we strip leading "v" and "V", while we
currently only prepend a lower case "v" when parsing a snapshot file
name. This results in broken snapshot links for tags that start with an
upper case "V". Avoid this by prepending a "V" as a fallback.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>