hg: Enstricten `base` revset alias
hg: Enstricten `base` revset alias

In 91f7cb1d1edcff I went into great detail why my "base" alias wasn't
really the merge base (tl;dr: `histedit` doesn't want the merge base but
rather the first commit to be edited, unlike Git).

So, it turns out that `histedit` will accept multiple revs, so you don't
actually need to specify the "first child of the merge base" like I was
doing.

In fact, you can just type `hg histedit` to edit all unpublished commits
on a topic branch. This works (at least, it works at work) because of an
undocumented option called `histedit.defaultrev` which is set to
`limit(only(.) & draft(), 50)`. In other words, "commits that are
reachable only from my current tip, which are unpublished, and btw,
no more than 50 of them in case you end up running this somewhere
silly".

So, I don't need my `base` alias anymore, but I am going to correct it
(ie. make it really the base) and leave it around for posterity.

wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent