Perform a version control merge operation.
You must be visiting a version controlled file, or in a `vc-dir' buffer.
On a distributed version control system, this runs a "merge"
operation to incorporate changes from another branch onto the
current branch, prompting for an argument list.
On a non-distributed version control system, this merges changes
between two revisions into the current fileset. This asks for
two revisions to merge from in the minibuffer. If the first
revision is a branch number, then merge all changes from that
branch. If the first revision is empty, merge the most recent
changes from the current branch.
(fn)