Fixed the encoding problem.

Pointed out by https://twitter.com/bleis/status/489596069416681472
The changelog is written as UTF-8, so we need to read it back as such.
See GitSCM.computeChangeLog()
Adjusted help for 'branches' param

...to latest changes in git-client plugin.
https://github.com/jenkinsci/git-client-plugin/pull/135
Update git-client plugin dependency to 1.10.0
Merge pull request #244 from alexanderlink/git-client-PR135-docu

Adjusted help for 'branches' param
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