Add a new function in the git driver

This is the third step of story #6971: Git online commit in the Tuleap UI
    - All basic functions to permform an online commit are now available

Change-Id: Ib9ee038a3b248940611a7a1360a2a9c73d53f773
Merge commit 'refs/changes/24/2524/39' of ssh://gerrit.tuleap.net:29418/tuleap into HEAD

* ssh://gerrit.tuleap.net:29418/tuleap:
  Add a new function in the git driver

Change-Id: If44ca8a3397e5f1198da1645e413b3ec5379924a
This is Tuleap 7.3.99.16

Change-Id: Iedd5944cd9d1ddc7855093c3506a4df3d3cdb43b