Update the current fileset or branch.
You must be visiting a version controlled file, or in a `vc-dir' buffer.
On a distributed version control system, this runs a "pull"
operation to update the current branch, prompting for an argument
list if required. Optional prefix ARG forces a prompt.
On a non-distributed version control system, update the current
fileset to the tip revisions. For each unchanged and unlocked
file, this simply replaces the work file with the latest revision
on its branch. If the file contains changes, any changes in the
tip revision are merged into the working file.
(fn &optional ARG)