Documentation: fix linkgit references

Documentation: fix linkgit references

There are a handful of incorrect "linkgit:<page>[<section>]"
instances in our documentation set.

 * Some have an extra colon after "linkgit:"; fix them by removing
   the extra colon;

 * Some refer to a page outside the Git suite, namely curl(1); fix
   them by using the `curl(1)` that already appears on the same page
   for the same purpose of referring the readers to its manual page.

 * Some spell the name of the page incorrectly, e.g. "rev-list" when
   they mean "git-rev-list"; fix them.

 * Some list the manual section incorrectly; fix them to make sure
   they match what is at the top of the target of the link.

Signed-off-by: Junio C Hamano <gitster@pobox.com>