rebase--am: use --cherry-pick instead of --ignore-if-in-upstream

When using `git format-patch --ignore-if-in-upstream` we are only
allowed to give a single revision range.  In the next commit we will
want to add an additional exclusion revision in order to handle fork
points correctly, so convert `git-rebase--am` to use a symmetric
difference with `--cherry-pick --right-only`.

This does not change the result of the format-patch invocation, just how
we spell the arguments.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--am.sh

@@ -29,7 +29,13 @@ skip)
 	;;
 esac
 
-test -n "$rebase_root" && root_flag=--root
+if test -z "$rebase_root"
+	# this is now equivalent to ! -z "$upstream"
+then
+	revisions=$upstream...$orig_head
+else
+	revisions=$onto...$orig_head
+fi
 
 ret=0
 if test -n "$keep_empty"
@@ -38,14 +44,15 @@ then
 	# 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 "$revisions"
+	git cherry-pick ${gpg_sign_opt:+"$gpg_sign_opt"} --allow-empty \
+		--right-only "$revisions"
 	ret=$?
 else
 	rm -f "$GIT_DIR/rebased-patches"
 
-	git format-patch -k --stdout --full-index --ignore-if-in-upstream \
+	git format-patch -k --stdout --full-index --cherry-pick --right-only \
 		--src-prefix=a/ --dst-prefix=b/ --no-renames --no-cover-letter \
-		$root_flag "$revisions" >"$GIT_DIR/rebased-patches"
+		"$revisions" >"$GIT_DIR/rebased-patches"
 	ret=$?
 
 	if test 0 != $ret