Use target branch name in batch merge to allow multiple instances

[valentindavid]

A project can use multiple instances of marge bots assigned to different target branches. This allows dealing with different release branches in parallel.

However batches could not be used on multiple instances because they conflicted on the name of the branch created for the batch. By naming this branch after the target branch, then multiple batch branches can exist.

Fixes #267

[sg70]

@tclh123 Is there a chance that this PR gets merged? @valentindavid can you update your PR?

[tclh123]

@sg70 Yeah, I think this is good. Not sure if we want it to be configurable?

[qqshfox]

@valentindavid Thanks for the contribution. Changing the branch name for branch will be a breaking change. Would you please add a configuration that is disabled by default?