* repo => repository for more standard language.
* Delete 3 repeats of explaining the origin/upstream/local.
* Add some links.
* Update `git pull` language for rebase changes.
Delete trailing newlines from all files, except
tools/ci/success-http-headers.txt and tools/setup/dev-motd, where they
are significant, and static/third, where we want to stay close to
upstream.
Signed-off-by: Anders Kaseorg <anders@zulipchat.com>