[PATCH 04/10] checkout --conflict=diff3: add a label for ancestor

[Jonathan Nieder <jrnieder@gmail.com>]

git checkout --conflict=diff3 can be used to present conflicts hunks
including text from the common ancestor:

	<<<<<<< ours
	ourside
	|||||||
	original
	=======
	theirside
	>>>>>>> theirs

The added information can be very helpful for resolving a merge by
hand, and merge tools can usually understand it without trouble
because it looks like output from ‘diff3 -m’.

Unfortunately, not all can: ‘diff3’ includes a label for the merge
base on the ||||||| line and it seems some tools cannot parse conflict
hunks without such a label.  Humans could use help in interpreting the
output, too.  So mark the start of the text from the common ancestor
with the label “||||||| original”.

git rerere will not have trouble parsing this output, since instead of
looking for a newline, it looks for whitespace after the |||||||
marker.  No other code in git tries to parse conflict hunks.

Reported-by: Stefan Monnier <monnier@iro.umontreal.ca>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
---
Since “original” a good name for the common ancestor?  I also
considered “base” and “ancestor”; the latter is too jargon-y for my
taste, but “base” seems all right.

 builtin/checkout.c |    2 +-
 t/t7201-co.sh      |    4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/builtin/checkout.c b/builtin/checkout.c
index d67f809..c60c3e0 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -149,7 +149,7 @@ static int checkout_merged(int pos, struct checkout *state)
 	read_mmblob(&ours, active_cache[pos+1]->sha1);
 	read_mmblob(&theirs, active_cache[pos+2]->sha1);
 
-	status = ll_merge(&result_buf, path, &ancestor, NULL,
+	status = ll_merge(&result_buf, path, &ancestor, "original",
 			  &ours, "ours", &theirs, "theirs", 0);
 	free(ancestor.ptr);
 	free(ours.ptr);
diff --git a/t/t7201-co.sh b/t/t7201-co.sh
index d20ed61..ebfa5ca 100755
--- a/t/t7201-co.sh
+++ b/t/t7201-co.sh
@@ -481,7 +481,7 @@ test_expect_success 'checkout with --merge, in diff3 -m style' '
 	(
 		echo "<<<<<<< ours"
 		echo ourside
-		echo "|||||||"
+		echo "||||||| original"
 		echo original
 		echo "======="
 		echo theirside
@@ -525,7 +525,7 @@ test_expect_success 'checkout --conflict=diff3' '
 	(
 		echo "<<<<<<< ours"
 		echo ourside
-		echo "|||||||"
+		echo "||||||| original"
 		echo original
 		echo "======="
 		echo theirside
-- 
1.7.0