DEV: Fix an apparently "too modern" git command (#10894)

A follow-up to a follow-up. (6932a373a3 and 572da7a57b)
Our `discourse_test` Docker image uses git 2.20.1 released on Dec 15, 2018. It does not support `git branch --show-current`. (it was added in 2.22.0)

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/$(git branch --show-current)`
+        `cd #{path} && git branch -u origin/$(git rev-parse --abbrev-ref HEAD)`
       end
 
       path