filter-branch: introduce convenience function "skip_commit"

With this function, a commit filter can leave out unwanted commits
(such as temporary commits).  It does _not_ undo the changeset
corresponding to that commit, but it _skips_ the revision.  IOW
no tree object is changed by this.

If you like to commit early and often, but want to filter out all
intermediate commits, marked by "@@@" in the commit message, you can
now do this with

	git filter-branch --commit-filter '
		if git cat-file commit $GIT_COMMIT | grep '@@@' > /dev/null;
		then
			skip_commit "$@";
		else
			git commit-tree "$@";
		fi' newbranch

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-filter-branch.txt

@@ -112,6 +112,11 @@ OPTIONS
 As a special extension, the commit filter may emit multiple
 commit ids; in that case, ancestors of the original commit will
 have all of them as parents.
++
+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 gitlink:git-rebase[1] instead).
 
 --tag-name-filter <command>::
 	This is the filter for rewriting tag names. When passed,
@@ -209,24 +214,39 @@ To remove commits authored by "Darl McBribe" from the history:
 git filter-branch --commit-filter '
 	if [ "$GIT_AUTHOR_NAME" = "Darl McBribe" ];
 	then
-		shift;
+		skip_commit "$@";
-		while [ -n "$1" ];
-		do
-			shift;
-			echo "$1";
-			shift;
-		done;
 	else
 		git commit-tree "$@";
 	fi' HEAD
 ------------------------------------------------------------------------------
 
+Note that the changes introduced by the commits, and not reverted by
+subsequent commits, will still be in the rewritten branch. If you want
+to throw out _changes_ together with the commits, you should use the
+interactive mode of gitlink:git-rebase[1].
+
+The function 'skip_commits' is defined as follows:
+
+--------------------------
+skip_commit()
+{
+	shift;
+	while [ -n "$1" ];
+	do
+		shift;
+		map "$1";
+		shift;
+	done;
+}
+--------------------------
+
 The shift magic first throws away the tree id and then the -p
 parameters.  Note that this handles merges properly! In case Darl
 committed a merge between P1 and P2, it will be propagated properly
 and all children of the merge will become merge commits with P1,P2
 as their parents instead of the merge commit.
 
+
 To restrict rewriting to only part of the history, specify a revision
 range in addition to the new branch name.  The new branch name will
 point to the top-most revision that a 'git rev-list' of this range

git-filter-branch.sh

@@ -23,6 +23,20 @@ map()
 	fi
 }
 
+# if you run 'skip_commit "$@"' in a commit filter, it will print
+# the (mapped) parents, effectively skipping the commit.
+
+skip_commit()
+{
+	shift;
+	while [ -n "$1" ];
+	do
+		shift;
+		map "$1";
+		shift;
+	done;
+}
+
 # override die(): this version puts in an extra line break, so that
 # the progress is still visible
 

t/t7003-filter-branch.sh

@@ -138,13 +138,7 @@ test_expect_success "remove a certain author's commits" '
 	git-filter-branch -f --commit-filter "\
 		if [ \"\$GIT_AUTHOR_NAME\" = \"B V Uips\" ];\
 		then\
-			shift;\
+			skip_commit \"\$@\";
-			while [ -n \"\$1\" ];\
-			do\
-				shift;\
-				echo \"\$1\";\
-				shift;\
-			done;\
 		else\
 			git commit-tree \"\$@\";\
 		fi" removed-author &&