am --rebasing: get patch body from commit, not from mailbox

Rebasing a commit that contains a diff in the commit message results
in a failure with output such as

  First, rewinding head to replay your work on top of it...
  Applying: My cool patch.
  fatal: sha1 information is lacking or useless
  (app/controllers/settings_controller.rb).
  Repository lacks necessary blobs to fall back on 3-way merge.
  Cannot fall back to three-way merge.
  Patch failed at 0001 My cool patch.

The reason is that 'git rebase' without -p/-i/-m internally calls 'git
format-patch' and pipes the output to 'git am --rebasing', which has
no way of knowing what is a real patch and what is a commit message
that contains a patch.

Make 'git am' while in --rebasing mode get the patch body from the
commit object instead of extracting it from the mailbox.

Patch by Junio, test case and commit log message by Martin.

Reported-by: anikey <arty.anikey@gmail.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Martin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-am.sh

@@ -683,6 +683,7 @@ To restore the original branch and stop patching run \"\$cmdline --abort\"."
 			sed -e '1,/^$/d' >"$dotest/msg-clean"
 			echo "$commit" > "$dotest/original-commit"
 			get_author_ident_from_commit "$commit" > "$dotest/author-script"
+			git diff-tree --root --binary "$commit" >"$dotest/patch"
 		else
 			{
 				sed -n '/^Subject/ s/Subject: //p' "$dotest/info"

t/t3405-rebase-malformed.sh

@@ -1,6 +1,6 @@
 #!/bin/sh
 
-test_description='rebase should not insist on git message convention'
+test_description='rebase should handle arbitrary git message'
 
 . ./test-lib.sh
 
@@ -12,6 +12,11 @@ It has two paragraphs, but its first paragraph is not friendly
 to oneline summary format.
 EOF
 
+cat >G <<\EOF
+commit log message containing a diff
+EOF
+
+
 test_expect_success setup '
 
 	>file1 &&
@@ -19,8 +24,9 @@ test_expect_success setup '
 	git add file1 file2 &&
 	test_tick &&
 	git commit -m "Initial commit" &&
+	git branch diff-in-message
 
-	git checkout -b side &&
+	git checkout -b multi-line-subject &&
 	cat F >file2 &&
 	git add file2 &&
 	test_tick &&
@@ -28,6 +34,17 @@ test_expect_success setup '
 
 	git cat-file commit HEAD | sed -e "1,/^\$/d" >F0 &&
 
+	git checkout diff-in-message &&
+	echo "commit log message containing a diff" >G &&
+	echo "" >>G
+	cat G >file2 &&
+	git add file2 &&
+	git diff --cached >>G &&
+	test_tick &&
+	git commit -F G &&
+
+	git cat-file commit HEAD | sed -e "1,/^\$/d" >G0 &&
+
 	git checkout master &&
 
 	echo One >file1 &&
@@ -36,13 +53,20 @@ test_expect_success setup '
 	git commit -m "Second commit"
 '
 
-test_expect_success rebase '
+test_expect_success 'rebase commit with multi-line subject' '
 
-	git rebase master side &&
+	git rebase master multi-line-subject &&
 	git cat-file commit HEAD | sed -e "1,/^\$/d" >F1 &&
 
 	test_cmp F0 F1 &&
 	test_cmp F F0
 '
 
+test_expect_success 'rebase commit with diff in message' '
+	git rebase master diff-in-message &&
+	git cat-file commit HEAD | sed -e "1,/^$/d" >G1 &&
+	test_cmp G0 G1 &&
+	test_cmp G G0
+'
+
 test_done