[makepkg] Integrated VCS checkout, update and revision selection

This is a feature request for better VCS support.

E.g., for Mercurial:

vcs=('vim:hg:http://vim.googlecode.com/hg/vim') # array of 'dest:vcs:source'
rev_vim=1290

This would checkout revision 1290 of
http://vim.googlecode.com/hg/vim into $srcdir/vim:

hg clone -u "$rev" "$source" "$srcdir/$dest" || return 1

If vim already exists, pull new revisions from the source and
update the repository to rev 1290:

( cd "$srcdir/$dest"; hg pull; hg update -r "$rev" ) || return 1

A special option to makepkg would tell it to update the revisions
to the most recent. This is in contrast to what is done currently
(--holdver to *not* make it update the version.)