[PATCH 05/16] checkout --conflict=diff3: add a label for ancestor

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 is 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 “||||||| base”.

git rerere’s conflict identifiers are not affected: to parse conflict
hunks, rerere looks for whitespace after the ||||||| marker rather
than a newline, and to compute preimage ids, rerere has its own code
for creating conflict hunks.  No other code in git tries to parse
conflict hunks.

Requested-by: Stefan Monnier [off-list ref]
Signed-off-by: Jonathan Nieder <redacted>
---
Label changed.  Thanks for the feedback.

 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..d652b4c 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, "base",
 			  &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..981ae8f 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 "||||||| base"
 		echo original
 		echo "======="
 		echo theirside

@@ -525,7 +525,7 @@ test_expect_success 'checkout --conflict=diff3' '
 	(
 		echo "<<<<<<< ours"
 		echo ourside
-		echo "|||||||"
+		echo "||||||| base"
 		echo original
 		echo "======="
 		echo theirside

-- 
1.7.0