filter-branch: fix --prune-empty on parentless commits

Previously, the git_commit_non_empty_tree function would always pass any
commit with no parents to git-commit-tree, regardless of whether the
tree was nonempty.  The new commit would then be recorded in the
filter-branch revision map, and subsequent commits which leave the tree
untouched would be correctly filtered.

With this change, parentless commits with an empty tree are correctly
pruned, and an empty file is recorded in the revision map, signifying
that it was rewritten to "no commits."  This works naturally with the
parent mapping for subsequent commits.

Signed-off-by: Devin J. Pohly <djpohly@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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/t7003-filter-branch.sh

@@ -362,7 +362,7 @@ test_expect_success 'prune empty works even without index/tree filters' '
 	test_cmp expect actual
 '
 
-test_expect_failure '--prune-empty is able to prune root commit' '
+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 &&
@@ -371,7 +371,7 @@ test_expect_failure '--prune-empty is able to prune root commit' '
 	test_cmp expect actual
 '
 
-test_expect_failure '--prune-empty is able to prune entire branch' '
+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 &&