[RFC PATCH 0/4] deny push to current branch of non-bare repo

The short of it is that it's dangerous, we see people confused by it
(there was another one just yesterday), and it's a FAQ:

  http://git.or.cz/gitwiki/GitFaq#head-b96f48bc9c925074be9f95c0fce69bcece5f6e73

The FAQ even says "don't do this until you know what you are doing." So
the safety valve is configurable, so that those who know what they are
doing can switch it off.

And it's even on Sam's "UI improvements" list. :)

Patch 4/4 is the interesting one. 1/4 is a cleanup I saw while fixing
tests. 2/4 is a cleanup to prepare for 3/4. And 3/4 fixes a bunch of
tests which were inadvertently doing such a push (but didn't care
because they didn't look at the working directory).

-Peff