pull, fetch: add --set-upstream option

pull, fetch: add --set-upstream option

Add the --set-upstream option to git pull/fetch
which lets the user set the upstream configuration
(branch.<current-branch-name>.merge and
branch.<current-branch-name>.remote) for the current branch.

A typical use-case is:

    git clone http://example.com/my-public-fork
    git remote add main http://example.com/project-main-repo
    git pull --set-upstream main master

or, instead of the last line:

    git fetch --set-upstream main master
    git merge # or git rebase

This is mostly equivalent to cloning project-main-repo (which sets
upsteam) and then "git remote add" my-public-fork, but may feel more
natural for people using a hosting system which allows forking from
the web UI.

This functionality is analog to "git push --set-upstream".

Signed-off-by: Corentin BOMPARD <corentin.bompard@etu.univ-lyon1.fr>
Signed-off-by: Nathan BERBEZIER <nathan.berbezier@etu.univ-lyon1.fr>
Signed-off-by: Pablo CHABANNE <pablo.chabanne@etu.univ-lyon1.fr>
Signed-off-by: Matthieu Moy <git@matthieu-moy.fr>
Patch-edited-by: Matthieu Moy <git@matthieu-moy.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>