Used when you want to track the original rev_id, change_id, branch_id,
etc. each revision had in the source repository by adding a label.
Can be used to turn any piece of metadata in to a label.
Note that the fields
source_name, source_filebranch_id, source_branch_id,
are set by VCP to be the same value as the corresponding fields
without the source prefix (except source_filebranch_id, which is built
from the file name, rooted in the repository, and for cvs
repositories, the branch number in angle brackets.) These source_*
fields (intended to be immutable in vcp) should be used to make labels
rather than their mutable equivalents which may be changed via a vcp
There is no way to add labels only to selected revisions at this
time, but if you try to add a label for metadata that is undefined
or empty, it will not be added.