Merge branch 'maint'

Merge branch 'maint'

* maint:
  git-bisect: make "start", "good" and "skip" succeed or fail atomically
  git-am: cope better with an empty Subject: line
  Ignore leading empty lines while summarizing merges
  bisect: squelch "fatal: ref HEAD not a symref" misleading message
  builtin-apply: Show a more descriptive error on failure when opening a patch
  Clarify documentation of git-cvsserver, particularly in relation to git-shell