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 "explicit is better than implicit" option), 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.
The solution is to do "git push origin master" instead (but with the configured remote and branch names). --smcv