Support fetch my-remote my-branch:my-branch-style fetches

[famod]

See #390 (comment)

Meaning:

• given e.g. <referenceBranch>my-branch</referenceBranch> & fetchReferenceBranch enabled

• find respective remote tracking branch (via BranchConfig) or fall back to trial and error?

• fetch into the remote tracking branch

• do a (fast forward) ref update of the local branch

• decide whether this should also create the local branch if it not exists?

• decide whether to do all this also for baseBranch