Merge branch 'mg/log-decorate-HEAD' into maint

The "log --decorate" enhancement in Git 2.4 that shows the commit
at the tip of the current branch e.g. "HEAD -> master", did not
work with --decorate=full.

* mg/log-decorate-HEAD:
  log: do not shorten decoration names too early
  log: decorate HEAD with branch name under --decorate=full, too