Document init script behavior when multiple remotes have master

When master is locally absent and more than one remote has it,
"git checkout master --" refuses to disambiguate (even if those
remote-tracking branches all agree on a commit), so the script falls
back to creating master at HEAD. The reflog populated by the resets
below then traces HEAD's history rather than a remote master's. This
is harmless -- master is reset to __testing_point__ either way -- but
unintuitive. Add a comment so a reader of the script does not have
to discover this from a confusing run.

Closes #2145.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: EliahKagan

init-tests-after-clone.sh

@@ -39,7 +39,12 @@ fi
 # This also keeps track of where we are, so we can get back here.
 git tag __testing_point__
 
-# The tests need a branch called master.
+# The tests need a branch called master. If master is locally absent but more
+# than one remote has it, "git checkout master --" fails (even if all those
+# remote-tracking branches point to the same commit), and we fall back to
+# creating master at HEAD. The reflog from the resets below then traces HEAD's
+# history rather than a remote master's, but master is reset to
+# __testing_point__ either way.
 git checkout master -- || git checkout -b master
 
 # The tests need a reflog history on the master branch.