git-rebase: add keep_empty flag

Add a command line switch to git-rebase to allow a user the ability to specify
that they want to keep any commits in a series that are empty.

When git-rebase's type is am, then this option will automatically keep any
commit that has a tree object identical to its parent.

This patch changes the default behavior of interactive rebases as well.  With
this patch, git-rebase -i will produce a revision set passed to
git-revision-editor, in which empty commits are commented out.  Empty commits
may be kept manually by uncommenting them.  If the new --keep-empty option is
used in an interactive rebase the empty commits will automatically all be
uncommented in the editor.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--am.sh

@@ -20,11 +20,20 @@ esac
 
 test -n "$rebase_root" && root_flag=--root
 
-git format-patch -k --stdout --full-index --ignore-if-in-upstream \
-	--src-prefix=a/ --dst-prefix=b/ \
-	--no-renames $root_flag "$revisions" |
-git am $git_am_opt --rebasing --resolvemsg="$resolvemsg" &&
-move_to_original_branch
+if test -n "$keep_empty"
+then
+	# we have to do this the hard way.  git format-patch completely squashes
+	# empty commits and even if it didn't the format doesn't really lend
+	# itself well to recording empty patches.  fortunately, cherry-pick
+	# makes this easy
+	git cherry-pick --allow-empty "$revisions"
+else
+	git format-patch -k --stdout --full-index --ignore-if-in-upstream \
+		--src-prefix=a/ --dst-prefix=b/ \
+		--no-renames $root_flag "$revisions" |
+	git am $git_am_opt --rebasing --resolvemsg="$resolvemsg"
+fi && move_to_original_branch
+
 ret=$?
 test 0 != $ret -a -d "$state_dir" && write_basic_state
 exit $ret