DEV: Fix a spec incompatibility with pre-2.28 git (#10892)

Regression introduced in 6932a373a3

spec/components/version_spec.rb

@@ -106,7 +106,7 @@ describe Discourse::VERSION do
 
         # Simulate a remote upstream
         `cd #{path} && git remote add origin #{path}/.git && git fetch -q`
-        `cd #{path} && git branch -u origin/main`
+        `cd #{path} && git branch -u origin/$(git branch --show-current)`
       end
 
       path

spec/support/helpers.rb

@@ -165,7 +165,7 @@ module Helpers
 
   def setup_git_repo(files)
     repo_dir = Dir.mktmpdir
-    `cd #{repo_dir} && git init . --initial-branch=main`
+    `cd #{repo_dir} && git init .`
     `cd #{repo_dir} && git config user.email 'someone@cool.com'`
     `cd #{repo_dir} && git config user.name 'The Cool One'`
     `cd #{repo_dir} && git config commit.gpgsign 'false'`