[RFC] Triangular Workflow: user friendly full implementation

[Jordan DE GEA <jordan.de-gea@grenoble-inp.org>]

We are working on full implementation of triangular workflow feature.
For now, the main options available are: 
	 - branch.<name>.pushRemote
	 - remote.pushDefault
And only setable by hands. 

As it can be difficult to understand, here is what we want to do. 


Context:
	- One main remote repository, e.g. git/git.
	- A remote fork (e.g. a GitHub fork) of git/git, e.g. me/git.
	- A local clone of me/git on the machine

Purposes:
	- the local branch master has to fetch to git/git by default
	- the local branch master has to push to me/git by default

Configuration wanted:
	- Add a remote to git/git e.g. `git remote add ...`
	- Set the fetch remote for branch as default. 

For now, we can do that by setting: 
	- branch.<name>.remote to git/git
	- branch.<name>.pushRemote to me/git
But many options set `branch.<name>.remote`, a suitable solution is to
implement an option for the fetch for example. 


Here is what we want to implement: 

1. 
	a. add the config var: remote.fetchDefault
	b. add the config var: branch.<name>.fetchRemote
	c. add `git fetch --set-default` in order to set remote.fetchDefault
	d. add `git fetch --set-remote` in order to set branch.<name>.fetchRemote
	e. add `git pull --set-default` in order to set remote.fetchDefault
	f. add `git pull --set-remote` in order to set branch.<name>.fetchRemote

2. 
	a. add `git push --set-default` in order to set remote.pushDefault
	b. add `git push --set-remote` in order to set branch.<name>.pushRemote


What's your opinion about this feature ?