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
Merge pull request #232 from alexanderlink/branchSpecSCMTriggerTests

SCMTrigger tests for various branch specs