Add option to choose branch to merge (not just master)

[adrian1001]

Features

• Added an optional, third column to the input file where the branch to be merged can be specified (split_line() needed adaption for this)

Fixes

• filter-branch aborted on empty commits; fixed by pruning empty commits first

[adrian1001]

Just realized that @robkub ran into the same problem (filter-branch failing on empty commits).
His/her solution seems better though - my approach is just a workaround.

For demonstration:
Try to merge https://github.com/adrian1001/test-repo-empty-commit with your version of git-join-repos: Fails with "mv: cannot stat '.git-rewrite/t/../index.new': No such file or directory"