rebase --keep-empty: always use interactive rebase

rebase --merge accepts --keep-empty but just ignores it, by using an
implicit interactive rebase the user still gets the rename detection
of a merge based rebase but with with --keep-empty support.

If rebase --keep-empty without --interactive or --merge stops for the
user to resolve merge conflicts then 'git rebase --continue' will
fail. This is because it uses a different code path that does not
create $git_dir/rebase-apply. As rebase --keep-empty was implemented
using cherry-pick it has never supported the am options and now that
interactive rebases support --signoff there is no loss of
functionality by using an implicit interactive rebase.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--am.sh

@@ -38,59 +38,47 @@ else
 fi
 
 ret=0
-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 ${gpg_sign_opt:+"$gpg_sign_opt"} --allow-empty \
-		$allow_rerere_autoupdate --right-only "$revisions" \
-		${restrict_revision+^$restrict_revision}
-	ret=$?
-else
-	rm -f "$GIT_DIR/rebased-patches"
+rm -f "$GIT_DIR/rebased-patches"
 
-	git format-patch -k --stdout --full-index --cherry-pick --right-only \
-		--src-prefix=a/ --dst-prefix=b/ --no-renames --no-cover-letter \
-		--pretty=mboxrd \
-		$git_format_patch_opt \
-		"$revisions" ${restrict_revision+^$restrict_revision} \
-		>"$GIT_DIR/rebased-patches"
-	ret=$?
+git format-patch -k --stdout --full-index --cherry-pick --right-only \
+	--src-prefix=a/ --dst-prefix=b/ --no-renames --no-cover-letter \
+	--pretty=mboxrd \
+	$git_format_patch_opt \
+	"$revisions" ${restrict_revision+^$restrict_revision} \
+	>"$GIT_DIR/rebased-patches"
+ret=$?
 
-	if test 0 != $ret
-	then
-		rm -f "$GIT_DIR/rebased-patches"
-		case "$head_name" in
-		refs/heads/*)
-			git checkout -q "$head_name"
-			;;
-		*)
-			git checkout -q "$orig_head"
-			;;
-		esac
+if test 0 != $ret
+then
+	rm -f "$GIT_DIR/rebased-patches"
+	case "$head_name" in
+	refs/heads/*)
+		git checkout -q "$head_name"
+		;;
+	*)
+		git checkout -q "$orig_head"
+		;;
+	esac
 
-		cat >&2 <<-EOF
+	cat >&2 <<-EOF
 
-		git encountered an error while preparing the patches to replay
-		these revisions:
+	git encountered an error while preparing the patches to replay
+	these revisions:
 
-		    $revisions
+	    $revisions
 
-		As a result, git cannot rebase them.
-		EOF
-		return $ret
-	fi
+	As a result, git cannot rebase them.
+	EOF
+	return $ret
+fi
 
-	git am $git_am_opt --rebasing --resolvemsg="$resolvemsg" \
-		--patch-format=mboxrd \
-		$allow_rerere_autoupdate \
-		${gpg_sign_opt:+"$gpg_sign_opt"} <"$GIT_DIR/rebased-patches"
-	ret=$?
+git am $git_am_opt --rebasing --resolvemsg="$resolvemsg" \
+	--patch-format=mboxrd \
+	$allow_rerere_autoupdate \
+	${gpg_sign_opt:+"$gpg_sign_opt"} <"$GIT_DIR/rebased-patches"
+ret=$?
 
-	rm -f "$GIT_DIR/rebased-patches"
-fi
+rm -f "$GIT_DIR/rebased-patches"
 
 if test 0 != $ret
 then

git-rebase.sh

@@ -452,6 +452,11 @@ then
 	test -z "$interactive_rebase" && interactive_rebase=implied
 fi
 
+if test -n "$keep_empty"
+then
+	test -z "$interactive_rebase" && interactive_rebase=implied
+fi
+
 if test -n "$interactive_rebase"
 then
 	type=interactive

t/t3421-rebase-topology-linear.sh

@@ -199,7 +199,7 @@ test_run_rebase () {
 	"
 }
 test_run_rebase success ''
-test_run_rebase failure -m
+test_run_rebase success -m
 test_run_rebase success -i
 test_run_rebase failure -p
 
@@ -214,7 +214,7 @@ test_run_rebase () {
 	"
 }
 test_run_rebase success ''
-test_run_rebase failure -m
+test_run_rebase success -m
 test_run_rebase success -i
 test_run_rebase failure -p