[rubygems/rubygems] Force "master" to be used regardless of `init.defaultBranch` config

Otherwise, if you have `init.defaultBranch main` configured, like I do,
a bunch of tests fail with things like:

```
============================================================================================================================================================================================================
Error: test_checkout_submodules(TestGemSourceGit): Gem::Exception: unable to find reference master in /Users/deivid/Code/rubygems/rubygems/tmp/test_rubygems_20220928-13878-xog1je/git/a
/Users/deivid/Code/rubygems/rubygems/lib/rubygems/source/git.rb:188:in `rev_parse'
/Users/deivid/Code/rubygems/rubygems/lib/rubygems/source/git.rb:143:in `dir_shortref'
/Users/deivid/Code/rubygems/rubygems/lib/rubygems/source/git.rb:158:in `install_dir'
/Users/deivid/Code/rubygems/rubygems/lib/rubygems/source/git.rb:94:in `checkout'
/Users/deivid/Code/rubygems/rubygems/test/rubygems/test_gem_source_git.rb:78:in `test_checkout_submodules'
     75:       system @git, "commit", "--quiet", "-m", "add submodule b"
     76:     end
     77:
  => 78:     source.checkout
     79:
     80:     assert_path_exist File.join source.install_dir, "a.gemspec"
     81:     assert_path_exist File.join source.install_dir, "b/b.gemspec"
============================================================================================================================================================================================================
fatal: ambiguous argument 'master': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
fatal: ambiguous argument 'master': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
E
```

In the future, I'd like to change things to use `main`, but the
straighforward fix now is to keep "master" and make the running
environment's git configuration not get in the middle.

https://github.com/rubygems/rubygems/commit/b09b1416f1

1 files changed, 1 insertions, 0 deletions

diff --git a/test/rubygems/helper.rb b/test/rubygems/helper.rb
index 258107bf04..8c959bae9c 100644
--- a/test/rubygems/helper.rb
+++ b/test/rubygems/helper.rb
@@ -575,6 +575,7 @@ class Gem::TestCase < Test::Unit::TestCase
     Dir.chdir directory do
       unless File.exist? ".git"
         system @git, "init", "--quiet"
+        system @git, "checkout", "-b", "master", "--quiet"
         system @git, "config", "user.name",  "RubyGems Tests"
         system @git, "config", "user.email", "rubygems@example"
       end