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