tigefa4u projects Trac

As a side effect, I had to make the INTERTRAC_SCHEME non-greedy,
because common shortcuts like `[trac2781]` were interpreted as
`tr` prefix followed by an `ac2781` revision...
Anyway, the greediness was not needed anymore, as I removed
the support for digits in the INTERTRAC_SCHEME

In addition, the TracMercurial plugin reintroduces support for
the `cset:`, `chgset:`, `tag:` and `branch:` link resolvers.
* the first 3 are synonyms and can be used to refer to a
changeset using either a revision hash, a revision
number or a tag name
* the last one is special, as it will refer to the
current ''head'' for that named branch