Merge branch 'jn/test-sanitize-git-env'

* jn/test-sanitize-git-env:
tests: scrub environment of GIT_* variables
config: drop support for GIT_CONFIG_NOGLOBAL
gitattributes: drop support for GIT_ATTR_NOGLOBAL
tests: suppress system gitattributes
tests: stop worrying about obsolete environment variables

diff --cc cache.h
Simple merge

diff --cc config.c
Simple merge

diff --cc t/test-lib.sh
Simple merge