Merge branch 'jc/show-usage-help'

The help text from "git $cmd -h" appear on the standard output for
some $cmd and the standard error for others.  The built-in commands
have been fixed to show them on the standard output consistently.

* jc/show-usage-help:
  builtin: send usage() help text to standard output
  oddballs: send usage() help text to standard output
  builtins: send usage_with_options() help text to standard output
  usage: add show_usage_if_asked()
  parse-options: add show_usage_with_options_if_asked()
  t0012: optionally check that "-h" output goes to stdout

diff --cc builtin/am.c
Simple merge

diff --cc builtin/credential.c
Simple merge

diff --cc builtin/gc.c
Simple merge

diff --cc builtin/index-pack.c
Simple merge

diff --cc builtin/rev-list.c
Simple merge

diff --cc builtin/unpack-objects.c
Simple merge

diff --cc builtin/var.c
Simple merge

diff --cc parse-options.c
Simple merge