chg: import frontend sources These files are copied from https://bitbucket.org/yuja/chg/ -r f897faa79687

debugrevlog: fix dumping manifest fails on empty first revision (issue5062)

help: don't crash in keyword search if an extension fails to provide docs Not all external extensions provide docs; if you use such an extension, you will experience a crash if you use "hg help --keyword <word>", and <word> happens to match the extension name.