[PATCH 0 of 2 RFC] Shell alias support

[Steve Losh]

This is a second version of a patch I sent a day or two ago to add shell
aliases to Mercurial, like so:

    [alias]
    qempty = !hg qrefresh -X "`hg root`"

This would help get rid of hacky "alias hgsomething='hg something'" aliases in
people's .whateverrc files.

A few issues brought up in IRC:

* There's no guarantee that the 'hg' command in the alias refers to the same
  'hg' script that is being used. For example, if you 'make local' and run
  "./hg qempty" the qrefresh command inside that alias will be run with the
  'hg' on your path, not the local one.

* The second patch (which adds "$@" expansion) simply replaces raw strings --
  it doesn't take quoting into account. This might be surprising, but makes the
  code much, much cleaner and makes quoting in the alias itself simpler:
  whatever is in the definition is exactly what will be passed to the shell.

* If this is something people aren't comfortable with it could be turned into
  a 'shellalias' extension instead, but I don't know how that would affect
  quoting in the .hgrc file.

* The '!' prefix was chosen because that's what git uses, but could be changed
  if desired.