tests: set old git default branch name for compatibility

Git's default branch name has changed on my machine (from "master" to
"main"). Let's set the old name in our tests so we're compatible with
both defaults (and maybe still compatible with Git versions that don't
know about the config option).

Differential Revision: https://phab.mercurial-scm.org/D9470

tests/test-git-interop.t

@@ -30,6 +30,7 b' Test auto-loading extension works:'
   [255]
   $ git init
   Initialized empty Git repository in $TESTTMP/nogit/.git/
+  $ git config --global init.defaultBranch master
 This status invocation shows some hg gunk because we didn't use
 `hg init --git`, which fixes up .git/info/exclude for us.
   $ hg status