Document init script behavior when multiple remotes have master

When `master` is locally absent and more than one remote has it,
`git checkout master --` fails by default even if all remotes
agree, and the script falls back to creating `master` at `HEAD`.
The reflog populated by the subsequent resets then traces `HEAD`'s
history rather than a remote `master`'s. This is harmless, because
`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.

This fixes #2145.

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

Author: EliahKagan

init-tests-after-clone.sh

@@ -40,6 +40,11 @@ fi
 git tag __testing_point__
 
 # The tests need a branch called master.
+#
+# If master is locally absent but more than one remote has it, checkout fails
+# by default even if all remotes agree, and we fall back to creating it at
+# HEAD. The reflog we populate 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.