Allow branches that begin with a number

[fwyzard]

Tighten the logic to distinguish a branch from a pull request: assume the branch argument to be a pull request only if the full argument is a decimal number, not if oit only begins with a number.

[cmsbuild]

A new Pull Request was created by @fwyzard (Andrea Bocci) for branch master.

@cmsbuild, @smuzaffar, @gudrutis, @mrodozov can you please review it and eventually sign? Thanks. You can sign-off by replying to this message having '+1' in the first line of your reply. You can reject by replying to this message having '-1' in the first line of your reply.

external issue cms-sw/cmsdist#3749

[smuzaffar]

looks good