RefSpec: reject refs ending in '/'

RefSpec: reject refs ending in '/'

We had a case in Gerrits superproject subscriptions where
'refs/heads/' was configured with the intention to mean 'refs/heads/*'.

The first expression lacks the '*', which is why it is not considered
a wildcard but it was considered valid and so was not found early to be
a typo.

Refs are not allowed to end with '/' anyway, so add a check for that.

Change-Id: I3ffdd9002146382acafb4fbc310a64af4cc1b7a9
Signed-off-by: Stefan Beller <sbeller@google.com>