theia: [git] Permission denied issues when executing `pull` and `push`

Moved from: https://github.com/theia-ide/theia/issues/583#issuecomment-336096605 Original message from @marcdumais-work:

I am trying the git extension using my Theia development environment. I have a few remotes defined. ATM I see that for pull and push operations, a random remote is picked (1st returned by “git remote” ?). It happens to be one I do not have permissions to push-to, and that doesn’t contain the branch I am working on, so push and pull operations fail. Fetch doesn’t seem to do anything and outputs no error message.

I guess these problems will be addressed in this issue by permitting to pick a specific remote for the operations?

push:

The command `git push antoine task` exited with an unexpected code: 1. The caller should either handle this error, or expect that exit code.
To github.com:hexa00/theia.git
 ! [remote rejected]   task -> task (permission denied)
error: failed to push some refs to 'git@github.com:hexa00/theia.git'

pull:

The command `git pull antoine` exited with an unexpected code: 1. The caller should either handle this error, or expect that exit code.
You asked to pull from the remote 'antoine', but did not specify
a branch. Because this is not the default configured remote
for your current branch, you must specify a branch on the command line.

PR: #650.