doc: add repo/ to .hgignore and .hgtags
These names were disappearing in the asciidoc output and no form of
escaping seems to help. Let's just add repo/ to make it explicit that
they're in the repository root.
Basic install:
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
See http://www.selenic.com/mercurial/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.