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.)