reachable.c: add HEAD to reachability starting commits

HEAD is not explicitly used as a starting commit for
calculating reachability, so if it's detached and reflogs
are disabled it may be pruned.

Add tests which demonstrate it. Test 'prune: prune former HEAD after checking
out branch' also reverts changes to repository.
Signed-off-by: NMax Kirillov <max@max630.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

reachable.c

@@ -229,6 +229,9 @@ void mark_reachable_objects(struct rev_info *revs, int mark_reflog,
 	/* Add all external refs */
 	for_each_ref(add_one_ref, revs);
 
+	/* detached HEAD is not included in the list above */
+	head_ref(add_one_ref, revs);
+
 	/* Add all reflog info */
 	if (mark_reflog)
 		for_each_reflog(add_one_reflog, revs);

t/t5304-prune.sh

@@ -104,6 +104,28 @@ test_expect_success 'prune: prune unreachable heads' '
 
 '
 
+test_expect_success 'prune: do not prune detached HEAD with no reflog' '
+
+	git checkout --detach --quiet &&
+	git commit --allow-empty -m "detached commit" &&
+	# verify that there is no reflogs
+	# (should be removed and disabled by previous test)
+	test ! -e .git/logs &&
+	git prune -n >prune_actual &&
+	: >prune_expected &&
+	test_cmp prune_actual prune_expected
+
+'
+
+test_expect_success 'prune: prune former HEAD after checking out branch' '
+
+	head_sha1=$(git rev-parse HEAD) &&
+	git checkout --quiet master &&
+	git prune -v >prune_actual &&
+	grep "$head_sha1" prune_actual
+
+'
+
 test_expect_success 'prune: do not prune heads listed as an argument' '
 
 	: > file2 &&