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.

doc: correct example concerning "hg purge" alias in man page "hgrc.5" The "hg purge" alias as currently described in "hgrc.5" only works, if the caller's current working directory is identical to the repository's root directory. This patch slightly modifies the example by adding an empty pattern as a file argument to the "hg status" command, thus forcing this command to list the affected files relative to the current directory.