[dotfiles] alias: add an alias to launch a specific Emacs just for mail.

Using mu and mu4e to read and filter mails.
[dotfiles] hgrc: re-organize HG aliases, add a few and fix some.

New:
 - 'p' and 'n' for 'next' and 'previous'
Fix:
 - 'pdiff' for the diff of the previous commit (parent)
 - 'odiff' with the direct previous amend (aka precursor)