It is easy to add incorrect "linkgit:<page>[<section>]" references
to our documentation suite. Catch these common classes of errors:
* Referring to Documentation/<page>.txt that does not exist.
* Referring to a <page> outside the Git suite. In general, <page>
must begin with "git".
* Listing the manual <section> incorrectly. The first line of the
Documentation/<page>.txt must end with "(<section>)".
with a new script "ci/lint-gitlink", and drive it from "make check-docs".
Signed-off-by: Junio C Hamano <gitster@pobox.com>
