cmdutil: use None as default value for "function pointer" instead of False Less type confusion.

graft: make skip messages more helpful This makes it possible for the user to understand and accept or work around what is happening.

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.)

convert: introduce hg.revs to replace hg.startrev and --rev with a revset The existing knobs for controlling which revisions to convert were often insufficient. Revsets is a shiny hammer that provides a better solution. Revsets has been introduced in --rev handling in a lot of other places while being more or less backwards compatible. Doing the same here would be a much more elegant ... but that would unfortunately not work in this case. "--rev 7" used to mean revision 0 to 7 - it would be an unacceptable change if it suddenly just meant revision 7. Instead we introduce a new configuration setting. It will only work for Mercurial repositories so adding a new commandline option for it would not be a nice solution. There is no way to use the fancy deprecation markup for configuration settings so we just remove the documentation of hg.startrev.