Merge branch 'dp/filter-branch-prune-empty' into maint

"git filter-branch --prune-empty" drops a single-parent commit that
becomes a no-op, but did not drop a root commit whose tree is empty.

* dp/filter-branch-prune-empty:
  p7000: add test for filter-branch with --prune-empty
  filter-branch: fix --prune-empty on parentless commits
  t7003: ensure --prune-empty removes entire branch when applicable
  t7003: ensure --prune-empty can prune root commit

Documentation/git-filter-branch.txt

@@ -167,14 +167,12 @@ to other tags will be rewritten to point to the underlying commit.
 	project root. Implies <<Remap_to_ancestor>>.
 
 --prune-empty::
-	Some kind of filters will generate empty commits, that left the tree
-	untouched.  This switch allow git-filter-branch to ignore such
-	commits.  Though, this switch only applies for commits that have one
-	and only one parent, it will hence keep merges points. Also, this
-	option is not compatible with the use of `--commit-filter`. Though you
-	just need to use the function 'git_commit_non_empty_tree "$@"' instead
-	of the `git commit-tree "$@"` idiom in your commit filter to make that
-	happen.
+	Some filters will generate empty commits that leave the tree untouched.
+	This option instructs git-filter-branch to remove such commits if they
+	have exactly one or zero non-pruned parents; merge commits will
+	therefore remain intact.  This option cannot be used together with
+	`--commit-filter`, though the same effect can be achieved by using the
+	provided `git_commit_non_empty_tree` function in a commit filter.
 
 --original <namespace>::
 	Use this option to set the namespace where the original commits

git-filter-branch.sh

@@ -46,6 +46,8 @@ git_commit_non_empty_tree()
 {
 	if test $# = 3 && test "$1" = $(git rev-parse "$3^{tree}"); then
 		map "$3"
+	elif test $# = 1 && test "$1" = 4b825dc642cb6eb9a060e54bf8d69288fbee4904; then
+		:
 	else
 		git commit-tree "$@"
 	fi

t/perf/p7000-filter-branch.sh

@@ -16,4 +16,9 @@ test_perf 'noop filter' '
 	git filter-branch -f base..HEAD
 '
 
+test_perf 'noop prune-empty' '
+	git checkout --detach tip &&
+	git filter-branch -f --prune-empty base..HEAD
+'
+
 test_done

t/t7003-filter-branch.sh

@@ -313,6 +313,27 @@ test_expect_success 'Tag name filtering allows slashes in tag names' '
 	git cat-file tag X/2 > actual &&
 	test_cmp expect actual
 '
+test_expect_success 'setup --prune-empty comparisons' '
+	git checkout --orphan master-no-a &&
+	git rm -rf . &&
+	unset test_tick &&
+	test_tick &&
+	GIT_COMMITTER_DATE="@0 +0000" GIT_AUTHOR_DATE="@0 +0000" &&
+	test_commit --notick B B.t B Bx &&
+	git checkout -b branch-no-a Bx &&
+	test_commit D D.t D Dx &&
+	mkdir dir &&
+	test_commit dir/D dir/D.t dir/D dir/Dx &&
+	test_commit E E.t E Ex &&
+	git checkout master-no-a &&
+	test_commit C C.t C Cx &&
+	git checkout branch-no-a &&
+	git merge Cx -m "Merge tag '\''C'\'' into branch" &&
+	git tag Fx &&
+	test_commit G G.t G Gx &&
+	test_commit H H.t H Hx &&
+	git checkout branch
+'
 
 test_expect_success 'Prune empty commits' '
 	git rev-list HEAD > expect &&
@@ -341,6 +362,22 @@ test_expect_success 'prune empty works even without index/tree filters' '
 	test_cmp expect actual
 '
 
+test_expect_success '--prune-empty is able to prune root commit' '
+	git rev-list branch-no-a >expect &&
+	git branch testing H &&
+	git filter-branch -f --prune-empty --index-filter "git update-index --remove A.t" testing &&
+	git rev-list testing >actual &&
+	git branch -D testing &&
+	test_cmp expect actual
+'
+
+test_expect_success '--prune-empty is able to prune entire branch' '
+	git branch prune-entire B &&
+	git filter-branch -f --prune-empty --index-filter "git update-index --remove A.t B.t" prune-entire &&
+	test_path_is_missing .git/refs/heads/prune-entire &&
+	test_must_fail git reflog exists refs/heads/prune-entire
+'
+
 test_expect_success '--remap-to-ancestor with filename filters' '
 	git checkout master &&
 	git reset --hard A &&