[TDF] Fix ROOT-8975: TDF crashed when Defineing an existing branch

[eguiraud]

PR #75 in roottest adds a test for this case.

[phsft-bot]

Starting build on centos7/gcc49, mac1012/native, slc6/gcc49, slc6/gcc62, ubuntu14/native with flags -Dvc=OFF -Dimt=ON -Dccache=ON
How to customize builds

[phsft-bot]

Build failed on centos7/gcc49.
See console output.

Failing tests:

• projectroot.roottest.root.multicore.roottest_root_multicore_threadExecutor

[phsft-bot]

Build failed on slc6/gcc49.
See console output.

Errors:

• ERROR: Timeout after 10 minutes
• ERROR: Error fetching remote repo 'origin'
• stderr: error: RPC failed; result=18, HTTP code = 200
• ERROR: Error fetching remote repo 'origin'

[phsft-bot]

Build failed on slc6/gcc62.
See console output.

Errors:

• ERROR: Timeout after 10 minutes
• ERROR: Error fetching remote repo 'origin'
• stderr: error: RPC failed; result=18, HTTP code = 200
• ERROR: Error fetching remote repo 'origin'