convert: update source shamap when using filemap, just as when not using filemap
The reverse mapping was introduced in 2147a734dcf9 to make roundtrip
conversions possible ... but it did not work when using filemap.
Roundtrips with filemaps will of course only work flawlessly if inverse
mappings are used.
Especially, if a lossy convert mapping is used in one direction, then only
linear lines of development can be converted in the other direction. With this
constraint convert will do the right thing by assuming that excluded files
haven't been changed.)
A test case with general coverage of hg-hg roundtrips with filemap is added.
(There a cases where adding records of converted revisions to the shamap in the
source repository doesn't work - especially when converting the same repo to
several other repos and back. It would arguably be better if convert only
updated the shamaps in the target repo but read shamaps from both the source
and and target repo ... but that is a different story. Making the stuff we have
work consistently is step forward no matter what.)