git: explicitly specify the branch to push to origin

git's behaviour when doing "git push origin" is configurable, and the
default is going to change in 2.0. In particular, if you've set
push.default to "nothing", the regression test will warn:

fatal: You didn't specify any refspecs to push, and push.default
is "nothing".
'git push origin' failed:  at .../lib/IkiWiki/Plugin/git.pm line 220.