[Needs work] Allow specifying custom shelveset base for shelve command

[nanoant]

For example:

 git-tfs shelve "Experiment v1" experiments experiment-1

This will populate changes starting from experiments ref, rather than
last TFS check-in ref.

NOTE: While existing tests pass, I did not add any new tests, the reason being is that I have no idea how to Mock two refs, still I have tested it in real-life scenario and it works fine for me.

[nanoant]

Okay unfortunately this needs more work 😢:

1. It contains only files modified in given commit range, but each file also includes modification done before base ref, which is undesired behavior.
2. Commit message also contains messages from commits before base ref.

Therefore, please do not merge it yet! But if you have an idea how make it working, let me know.