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.
#!/bin/sh
hg init repo
cd repo
touch foo
hg ci -Am 'add foo'
hg up -C null
# this should be stored as a delta against rev 0
echo foo bar baz > foo
hg ci -Am 'add foo again'
hg debugindex .hg/store/data/foo.i