[arch-projects] [devtools] Minor cleanups of common.sh

[Luke Shumaker]

I'm the maintainer of the equivalent of devtools in Parabola, and I
realized that most of the changes we apply have never been pushed
upstream.  So I'm doing that now.

This is a set of minor improvements to common.sh that I hope are
fairly uncontroversial; even if they aren't directly taken advantage
of right now in devtools.

In the coming days, I will be sending more patches as I isolate out
many years of `git merge`s into sets of atomic commits that I think
you might welcome.

-- 
Happy hacking,
~ Luke Shumaker