Explain what each step in the init script achieves

This adds comments to init-tests-after-clone.sh to explain what
each of the steps is doing that is needed by some of the tests.

It also refactors some recently added logic, in a way that lends
itself to greater clarity when combined with these comments.

Author: EliahKagan

init-tests-after-clone.sh

@@ -13,14 +13,26 @@ if test -z "$TRAVIS"; then
     esac
 fi
 
+# Stop if we have run this. (You can delete __testing_point__ to let it rerun.)
+# 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.
 git checkout master -- || git checkout -b master
+
+# The tests need a reflog history on the master branch.
 git reset --hard HEAD~1
 git reset --hard HEAD~1
 git reset --hard HEAD~1
+
+# Point the master branch where we started, so we test the correct code.
 git reset --hard __testing_point__
 
-test -z "$TRAVIS" || exit 0  # CI jobs will already have taken care of the rest.
+# Do some setup that CI takes care of but that may not have been done locally.
+if test -z "$TRAVIS"; then
+    # The tests needs some version tags. Try to get them even in forks.
+    git fetch --all --tags
 
-git fetch --all --tags
-git submodule update --init --recursive
+    # The tests need submodules, including a submodule with a submodule.
+    git submodule update --init --recursive
+fi