gitmanager.py: don't force or require --global git config
gitmanager.py: don't force or require --global git config

SmokeDetector should not care whether this setting is global or not,
as long as the value is correct in the current working directory.

Forcing it to be set globally hampers work on private check-outs and
makes some tests very hard to write.

Charcoal-SE/SmokeDetector,Charcoal-SE/SmokeDetector