doc: add an UPSTREAM BRANCHES section to pull/push/fetch

From user feedback: one user mentioned that they don't know what the
term "upstream branch" means. As far as I can tell, the most complete
description is under the `--track` option in `git branch`. Upstreams
are an important concept in Git and the `git branch` man page is not an
obvious place for that information to live.

There's also a very terse description of "upstream branch" in the
glossary that's missing a lot of key information, like the fact that the
upstream is used by `git status` and `git pull`, as well as a
description in `git-config` in `branch.<name>.remote` which doesn't
explain the relationship to `git status` either.

Since the `git pull`, `git push`, and `git fetch` man pages already
include sections on REMOTES and the syntax for URLs, add a section on
UPSTREAM BRANCHES to `urls-remotes.adoc`.

In the new UPSTREAM BRANCHES section, cover the various ways that
upstreams branches are automatically set in Git, since users may
mistakenly think that their branch does not have an upstream branch if
they didn't explicitly set one.

A terminology note: Git uses two terms for this concept:

- "tracking" as in "the tracking information for the 'foo' branch"
  or the `--track` option to `git branch`
- "upstream" or "upstream branch", as in `git push --set-upstream`.
  This term is also used in the `git rebase` man page to refer to the
  first argument to `git rebase`, as well as in `git pull` to refer to
  the branch which is going to be merged into the current branch ("merge
  the upstream branch into the current branch")

Use "upstream branch" as a heading for this concept even though the term
"upstream branch" is not always used strictly in the sense of "the
tracking information for the current branch". "Upstream" is used much
more often than "tracking" in the Git docs to refer to this concept and
the goal is to help users understand the docs.

Signed-off-by: Julia Evans <julia@jvns.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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