[Proof of concept PATCH] implement --prune-empty switch for filter-branch

[Pierre Habouzit <madcoder@debian.org>]

This is not a real patch (lacks a test at least) and is absolutely not
tested, though should basically work as expected.

This is only meant as a basis for discussion.

Signed-off-by: Pierre Habouzit <madcoder@debian.org>
---
 Documentation/git-filter-branch.txt |   14 ++++++++++++++
 git-filter-branch.sh                |   26 +++++++++++++++++++++++++-
 2 files changed, 39 insertions(+), 1 deletions(-)

diff --git a/Documentation/git-filter-branch.txt b/Documentation/git-filter-branch.txt
index fed6de6..451950b 100644
--- a/Documentation/git-filter-branch.txt
+++ b/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'.
diff --git a/git-filter-branch.sh b/git-filter-branch.sh
index 8af3126..9c83402 100755
--- a/git-filter-branch.sh
+++ b/git-filter-branch.sh
@@ -49,6 +49,15 @@ die()
 	echo "$*" >&2
 	exit 1
 }
+
+git_commit_non_empty_tree()
+{
+	if test $# = 3 && test "$1" = $(git rev-parse "$3^{tree}"); then
+		map "$3"
+	else
+		git commit-tree "$@"
+	fi
+}
 EOF
 )
 
@@ -95,6 +104,7 @@ d=                      temporary path to use for rewriting
 f,force                 force filter-branch to run
 subdirectory-filter=    only look at the history touching that specific subdirectory
 original=               namespace where the original commits will be stored (default: refs/original)
+prune-empty             use this if you want to automatically prune empty commits
 
  Filters that you can run:
 
@@ -121,11 +131,12 @@ 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 "$1" in
@@ -138,6 +149,8 @@ do
 	--original)
 		orig_namespace=$(expr "$2/" : '\(.*[^/]\)/*$')/
 		shift;;
+	--prune-empty)
+		prune_empty=t;;
 
 	--env-filter)
 		filter_env="$2"; shift;;
@@ -161,6 +174,17 @@ do
 	shift
 done
 
+case "$prune_empty,$filter_commit" in
+',')
+	filter_commit='git commit-tree "$@"';;
+'t,')
+	filter_commit='git_commit_non_empty_tree "$@"';;
+','*)
+	;;
+*)
+	die "Cannot set --prune-empty and --filter-commit at the same time"
+esac
+
 case "$force" in
 t)
 	rm -rf "$tempdir"
-- 
1.6.0.3.758.gc29b0