git,git-toolfile: new external, version 1.8.3.1

Add GIT 1.8.3.1.

Signed-off-by: David Abdurachmanov <davidlt@cern.ch>
cmssw-tool-conf: add git as optional tool in SCRAM

GIT will not be activated automatically. Users can activate GIT
from CMSSW via scram:

    scram setup git

Signed-off-by: David Abdurachmanov <davidlt@cern.ch>