Merge branch 'mk/reachable-protect-detached-head' into maint

Reachability check (used in "git prune" and friends) did not add a
detached HEAD as a starting point to traverse objects still in use.

* mk/reachable-protect-detached-head:
  reachable.c: add HEAD to reachability starting commits

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 &&