fix #286: configure a home directory for git

[dsamx]

Having a home directory allows to store global git settings for the
service if required, for example this allows to store git credentials
if a generally used private git-backend requires a login which the
scm-service should use.
The only other solution would be to store the login credentials in the
_service file which is unacceptable.

[dsamx]

The python 2.7 build creates a warning:
"W: 18, 8: String statement has no effect (pointless-string-statement)"
I don't get it - if I add the os.environ setting then the git credentials work, so it is not a pointless statement.