Create a generic commit_to_patch method in the internal API

This method accept a git repo object and a commit object as input and
returns the patch corresponding to this commit.
Update the view_commit_patch endpoint to rely on commit_to_patch from the internal API
Adjust the commit_to_patch method in the internal API to handle multiple commits

This way we can generate a single patch file containing multiple commits
in it (useful for example for reviews).
When creating a pull-request we need to distinguish the from_branch to the target branch
Adjust the generic Request Pull link as it specifies the from_branch not the target branch
Add a new endpoint: request_pull_patch

This new endpoint returns all the commits in a single file that can be
directly applied with ``git am``
Adjust the javascript in the pull_request template to update properly the branch argument