filter-branch: add git_commit_non_empty_tree and --prune-empty.

git_commit_non_empty_tree is added to the functions that can be run from
commit filters. Its effect is to commit only commits actually touching the
tree and that are not merge points either.

The option --prune-empty is added. It defaults the commit-filter to
'git_commit_non_empty_tree "$@"', and can be used with any other
combination of filters, except --commit-hook that must used
'git_commit_non_empty_tree "$@"' where one puts 'git commit-tree "$@"'
usually to achieve the same result.

Signed-off-by: Pierre Habouzit <madcoder@debian.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-filter-branch.txt

@@ -122,6 +122,10 @@ You can use the 'map' convenience function in this filter, and other
 convenience functions, too.  For example, calling 'skip_commit "$@"'
 will leave out the current commit (but not its changes! If you want
 that, use 'git-rebase' instead).
++
+You can also use the 'git_commit_non_empty_tree "$@"' instead of
+'git commit-tree "$@"' if you don't wish to keep commits with a single parent
+and that makes no change to the tree.
 
 --tag-name-filter <command>::
 	This is the filter for rewriting tag names. When passed,
@@ -151,6 +155,16 @@ to other tags will be rewritten to point to the underlying commit.
 	The result will contain that directory (and only that) as its
 	project root.
 
+--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.
+
 --original <namespace>::
 	Use this option to set the namespace where the original commits
 	will be stored. The default value is 'refs/original'.

git-filter-branch.sh

@@ -40,6 +40,16 @@ skip_commit()
 	done;
 }
 
+# if you run 'git_commit_non_empty_tree "$@"' in a commit filter,
+# it will skip commits that leave the tree untouched, commit the other.
+git_commit_non_empty_tree()
+{
+	if test $# = 3 && test "$1" = $(git rev-parse "$3^{tree}"); then
+		map "$3"
+	else
+		git commit-tree "$@"
+	fi
+}
 # override die(): this version puts in an extra line break, so that
 # the progress is still visible
 
@@ -109,11 +119,12 @@ filter_tree=
 filter_index=
 filter_parent=
 filter_msg=cat
-filter_commit='git commit-tree "$@"'
+filter_commit=
 filter_tag_name=
 filter_subdir=
 orig_namespace=refs/original/
 force=
+prune_empty=
 while :
 do
 	case "$1" in
@@ -126,6 +137,11 @@ do
 		force=t
 		continue
 		;;
+	--prune-empty)
+		shift
+		prune_empty=t
+		continue
+		;;
 	-*)
 		;;
 	*)
@@ -176,6 +192,17 @@ do
 	esac
 done
 
+case "$prune_empty,$filter_commit" in
+,)
+	filter_commit='git commit-tree "$@"';;
+t,)
+	filter_commit="$functions;"' git_commit_non_empty_tree "$@"';;
+,*)
+	;;
+*)
+	die "Cannot set --prune-empty and --filter-commit at the same time"
+esac
+
 case "$force" in
 t)
 	rm -rf "$tempdir"

t/t7003-filter-branch.sh

@@ -262,4 +262,12 @@ test_expect_success 'Tag name filtering allows slashes in tag names' '
 	test_cmp expect actual
 '
 
+test_expect_success 'Prune empty commits' '
+	git rev-list HEAD > expect &&
+	make_commit to_remove &&
+	git filter-branch -f --index-filter "git update-index --remove to_remove" --prune-empty HEAD &&
+	git rev-list HEAD > actual &&
+	test_cmp expect actual
+'
+
 test_done