rerere: teach rerere to handle nested conflicts

Currently rerere can't handle nested conflicts and will error out when
it encounters such conflicts.  Do that by recursively calling the
'handle_conflict' function to normalize the conflict.

Note that a conflict like this would only be produced if a user
commits a file with conflict markers, and gets a conflict including
that in a susbsequent operation.

The conflict ID calculation here deserves some explanation:

As we are using the same handle_conflict function, the nested conflict
is normalized the same way as for non-nested conflicts, which means
the ancestor in the diff3 case is stripped out, and the parts of the
conflict are ordered alphabetically.

The conflict ID is however is only calculated in the top level
handle_conflict call, so it will include the markers that 'rerere'
adds to the output.  e.g. say there's the following conflict:

    <<<<<<< HEAD
    1
    =======
    <<<<<<< HEAD
    3
    =======
    2
    >>>>>>> branch-2
    >>>>>>> branch-3~

it would be recorde as follows in the preimage:

    <<<<<<<
    1
    =======
    <<<<<<<
    2
    =======
    3
    >>>>>>>
    >>>>>>>

and the conflict ID would be calculated as

    sha1(1<NUL><<<<<<<
    2
    =======
    3
    >>>>>>><NUL>)

Stripping out vs. leaving the conflict markers in place in the inner
conflict should have no practical impact, but it simplifies the
implementation.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/rerere.txt

@@ -138,3 +138,45 @@ SHA1('B<NUL>C<NUL>').
 If there are multiple conflicts in one file, the sha1 is calculated
 the same way with all hunks appended to each other, in the order in
 which they appear in the file, separated by a <NUL> character.
+
+Nested conflicts
+~~~~~~~~~~~~~~~~
+
+Nested conflicts are handled very similarly to "simple" conflicts.
+Similar to simple conflicts, the conflict is first normalized by
+stripping the labels from conflict markers, stripping the common ancestor
+version, and the sorting the conflict hunks, both for the outer and the
+inner conflict.  This is done recursively, so any number of nested
+conflicts can be handled.
+
+The only difference is in how the conflict ID is calculated.  For the
+inner conflict, the conflict markers themselves are not stripped out
+before calculating the sha1.
+
+Say we have the following conflict for example:
+
+    <<<<<<< HEAD
+    1
+    =======
+    <<<<<<< HEAD
+    3
+    =======
+    2
+    >>>>>>> branch-2
+    >>>>>>> branch-3~
+
+After stripping out the labels of the conflict markers, and sorting
+the hunks, the conflict would look as follows:
+
+    <<<<<<<
+    1
+    =======
+    <<<<<<<
+    2
+    =======
+    3
+    >>>>>>>
+    >>>>>>>
+
+and finally the conflict ID would be calculated as:
+`sha1('1<NUL><<<<<<<\n3\n=======\n2\n>>>>>>><NUL>')`

rerere.c

@@ -365,12 +365,18 @@ static int handle_conflict(struct strbuf *out, struct rerere_io *io,
 		RR_SIDE_1 = 0, RR_SIDE_2, RR_ORIGINAL
 	} hunk = RR_SIDE_1;
 	struct strbuf one = STRBUF_INIT, two = STRBUF_INIT;
-	struct strbuf buf = STRBUF_INIT;
+	struct strbuf buf = STRBUF_INIT, conflict = STRBUF_INIT;
 	int has_conflicts = -1;
 
 	while (!io->getline(&buf, io)) {
 		if (is_cmarker(buf.buf, '<', marker_size)) {
-			break;
+			if (handle_conflict(&conflict, io, marker_size, NULL) < 0)
+				break;
+			if (hunk == RR_SIDE_1)
+				strbuf_addbuf(&one, &conflict);
+			else
+				strbuf_addbuf(&two, &conflict);
+			strbuf_release(&conflict);
 		} else if (is_cmarker(buf.buf, '|', marker_size)) {
 			if (hunk != RR_SIDE_1)
 				break;

t/t4200-rerere.sh

@@ -601,4 +601,41 @@ test_expect_success 'rerere with unexpected conflict markers does not crash' '
 	git rerere clear
 '
 
+test_expect_success 'rerere with inner conflict markers' '
+	git reset --hard &&
+
+	git checkout -b A master &&
+	echo "bar" >test &&
+	git add test &&
+	git commit -q -m two &&
+	echo "baz" >test &&
+	git add test &&
+	git commit -q -m three &&
+
+	git reset --hard &&
+	git checkout -b B master &&
+	echo "foo" >test &&
+	git add test &&
+	git commit -q -a -m one &&
+
+	test_must_fail git merge A~ &&
+	git add test &&
+	git commit -q -m "will solve conflicts later" &&
+	test_must_fail git merge A &&
+
+	echo "resolved" >test &&
+	git add test &&
+	git commit -q -m "solved conflict" &&
+
+	echo "resolved" >expect &&
+
+	git reset --hard HEAD~~ &&
+	test_must_fail git merge A~ &&
+	git add test &&
+	git commit -q -m "will solve conflicts later" &&
+	test_must_fail git merge A &&
+	cat test >actual &&
+	test_cmp expect actual
+'
+
 test_done