Merge branch 'fc/atmark-in-refspec'

Merge branch 'fc/atmark-in-refspec'

"@" sometimes worked (e.g. "git push origin @:there") as a part of
a refspec element, but "git push origin @" did not work, which has
been corrected.

* fc/atmark-in-refspec:
  refspec: make @ a synonym of HEAD
  tests: push: trivial cleanup
  tests: push: improve cleanup of HEAD tests