[PATCH 08/10] checkout -m --conflict=diff3: add a label for ancestor

[Jonathan Nieder <jrnieder@gmail.com>]

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

The added information can be very helpful for resolving a merge by
hand, and many merge tools can usually grok it because it is very
similar to the output from ‘diff3 -m’.

Unfortunately, some tools reportedly cannot parse the conflict hunks
because of a small difference from diff3: diff3 -m includes a label
for the merge base on the ||||||| line.  Humans can benefit from a
cue when learning to interpreting the format, too.  So mark the start
of the text from the old branch with a label based on the branch’s
name.

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>
---
 builtin/checkout.c |    1 +
 t/t7201-co.sh      |    2 +-
 2 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/builtin/checkout.c b/builtin/checkout.c
index c60c3e0..ebc5891 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -439,6 +439,7 @@ static int merge_working_tree(struct checkout_opts *opts,
 			ret = reset_tree(new->commit->tree, opts, 1);
 			if (ret)
 				return ret;
+			o.ancestor = old->name;
 			o.branch1 = new->name;
 			o.branch2 = "local";
 			merge_trees(&o, new->commit->tree, work,
diff --git a/t/t7201-co.sh b/t/t7201-co.sh
index 970a460..ae14b19 100755
--- a/t/t7201-co.sh
+++ b/t/t7201-co.sh
@@ -209,7 +209,7 @@ test_expect_success 'checkout --merge --conflict=diff3 <branch>' '
 	a
 	c
 	e
-	|||||||
+	||||||| master
 	a
 	b
 	c
-- 
1.7.0