Do not refer anymore explicitly to BackendLogger

We should depend on abstraction not implementation.

Part of request #7592: Be more verbose in git logging

Change-Id: Id3dde05110ad655c9b40c57548e88d6901d06f7f
Merge commit 'refs/changes/08/3208/1' of ssh://gerrit.tuleap.net:29418/tuleap into HEAD

* ssh://gerrit.tuleap.net:29418/tuleap:
  Do not refer anymore explicitly to BackendLogger

Change-Id: I7c3357ae889853c41df1cd948623b09b83fa171d
This is Tuleap 7.6.99.43

Change-Id: I8e3a1226119cfa58d266c6cae1f839ad594c998a