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.