Merge branch 'jk/check-docs-update'

Simplify "make check-docs" implementation and update its coverage.

* jk/check-docs-update:
  check-docs: get documented command list from Makefile
  check-docs: drop git-help special-case
  check-docs: list git-gui as a command
  check-docs: factor out command-list
  command-list: mention git-credential-* helpers
  command-list: add git-sh-i18n
  check-docs: update non-command documentation list
  check-docs: mention gitweb specially

diff --cc Makefile
Simple merge