Fixed advanced branch spec behaviour in getCandidateRevisions

This commit relates to
https://github.com/jenkinsci/git-client-plugin/pull/129
Fixing expected GIT_BRANCH environment variable behaviour

See comments in https://github.com/jenkinsci/git-plugin/pull/225
Extracted getBranchName method - needed at 2nd location
Added branchSpec SCMTrigger behaviour tests
Added JGit and CliGit SCMTrigger tests
Added SCMTrigger tests with disabledRemotePolling
Speeding up SCMTrigger tests
Enable config round trip test again

Was commented out in a commit which did not describe why it was
removed.
Remove trailing whitespace lines from recent commits
Added missing url textbox and help.
Fixed all caps Descriptor name.
Added URL validation from GitBlit example.
Missing brace, egg on my face.