rebase: extract am code to new source file

Extract the code for am-based rebase to git-rebase--am.sh.

Suggested-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Martin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--am.sh

@@ -0,0 +1,34 @@
+#!/bin/sh
+#
+# Copyright (c) 2010 Junio C Hamano.
+#
+
+. git-sh-setup
+
+case "$action" in
+continue)
+	git am --resolved --3way --resolvemsg="$resolvemsg" &&
+	move_to_original_branch
+	exit
+	;;
+skip)
+	git am --skip -3 --resolvemsg="$resolvemsg" &&
+	move_to_original_branch
+	exit
+	;;
+esac
+
+test -n "$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
+ret=$?
+test 0 != $ret -a -d "$state_dir" &&
+	echo $head_name > "$state_dir/head-name" &&
+	echo $onto > "$state_dir/onto" &&
+	echo $orig_head > "$state_dir/orig-head" &&
+	echo "$GIT_QUIET" > "$state_dir/quiet"
+exit $ret