overlayworkingctx: invalidate the manifest cache when changing parents
This fixes problems noticeable when rebasing several commits into one
destination commit using ``--collapse``. The manifest cache needs to be cleared
each time.
Differential Revision: https://phab.mercurial-scm.org/D1244
#require docutils gettext
$ $TESTDIR/check-gendoc de
checking for parse errors
Die Dateien werden dem Projektarchiv beim n\xc3\xa4chsten \xc3\x9cbernehmen (commit) hinzugef\xc3\xbcgt. Um dies vorher r\xc3\xbcckg\xc3\xa4ngig zu machen, siehe:hg:`forget`. (esc)
warning: please have a space before :hg: