Merge branch 'maint'

Merge branch 'maint'

* maint:
  SunOS grep does not understand -C<n> nor -e
  Fix export_marks() error handling.
  git branch: clean up detached branch handling
  git branch: avoid unnecessary object lookups
  git branch: fix performance problem
  do_one_ref(): null_sha1 check is not about broken ref

Conflicts:
	Makefile