Merge branch 'maint'

Merge branch 'maint'

* maint:
  Unset NO_C99_FORMAT on Cygwin.
  Fix a "pointer type missmatch" warning.
  Fix some "comparison is always true/false" warnings.
  Fix an "implicit function definition" warning.
  Fix a "label defined but unreferenced" warning.
  Document the config variable format.suffix
  git-merge: fail correctly when we cannot fast forward.
  builtin-archive: use RUN_SETUP
  Fix git-gc usage note