builtin-status: submodule summary support

builtin-status: submodule summary support

This commit teaches 'git commit/status' show a new 'Modified submodules'
section, which is an output from:

  git submodule summary --cached --for-status --summary-limit <limit>

just before the 'Untracked files' section.

The <limit> is given by the config variable status.submodulesummary
to limit the submodule summary size. status.submodulesummary is a
bool/int variable with value:

  - false or 0 by default to disable the summary, or
  - positive number to limit the summary size, or
  - true or negative number to unlimit the summary size.

Also mention status.submodulesummary in the documentation.

Signed-off-by: Ping Yin <pkufranky@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>