tests: make test-subrepo-git.t compatible with git's master->main rename

Git is about to change the default branch from "master" to
"main". Recent versions has started warning about that, which makes
tests fail. I assume the tests would fail in a different way once the
default has changed. To make us compatible with that name change,
let's set configure the default to be "master". That value makes the
tests still work on older Git version (those where the default branch
name is not configurable).

Differential Revision: https://phab.mercurial-scm.org/D9746

tests/test-subrepo-git.t

@@ -17,6 +17,10 b' make git commits repeatable'
   $ GIT_COMMITTER_DATE="$GIT_AUTHOR_DATE"; export GIT_COMMITTER_DATE
   $ GIT_CONFIG_NOSYSTEM=1; export GIT_CONFIG_NOSYSTEM
 
+set default branch to value compatible with new and old git version
+
+  $ git config --global init.defaultBranch master
+
 root hg repo
 
   $ hg init t