test-subrepo-git: ignore global git config

This was causing a test failure for people with company-wide settings.
Still need a way to ignore local config.

tests/test-subrepo-git.t

@@ -10,6 +10,7 b' make git commits repeatable'
   $ GIT_COMMITTER_NAME="$GIT_AUTHOR_NAME"; export GIT_COMMITTER_NAME
   $ GIT_COMMITTER_EMAIL="$GIT_AUTHOR_EMAIL"; export GIT_COMMITTER_EMAIL
   $ GIT_COMMITTER_DATE="$GIT_AUTHOR_DATE"; export GIT_COMMITTER_DATE
+  $ GIT_CONFIG_NOSYSTEM=1; export GIT_CONFIG_NOSYSTEM
 
 root hg repo