tests: add test where copy source is deleted and added back
This shows another difference between pathcopies() and mergecopies():
mergecopies() considers files that have been deleted and then added
back as different files, but pathcopies() does not.
Differential Revision: https://phab.mercurial-scm.org/D6330
#require docutils gettext
$ $TESTDIR/check-gendoc it
checking for parse errors