makepkg: allow using GIT source URLs
Allow specifing GIT sources using the following syntax
source=('<folder>::<repo>#<fragment>')
This will download the git repo <repo> into <folder> (into $SRCDIR
if set, otherwise $startdir). <repo> must start with "git", but
non-git protocols are handled using (e.g.) "git+http://...".
The <fragment> can be used to specify a branch, tag, or commit to
build from. e.g. branch=maint.
Checksum entries for git sources should be "SKIP".
Signed-off-by: Allan McRae <allan@archlinux.org>