conflict-marker-size: add test and docs

Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/gitattributes.txt

@@ -525,6 +525,23 @@ When left unspecified, the driver itself is used for both
 internal merge and the final merge.
 
 
+`conflict-marker-size`
+^^^^^^^^^^^^^^^^^^^^^^
+
+This attribute controls the length of conflict markers left in
+the work tree file during a conflicted merge.  Only setting to
+the value to a positive integer has any meaningful effect.
+
+For example, this line in `.gitattributes` can be used to tell the merge
+machinery to leave much longer (instead of the usual 7-character-long)
+conflict markers when merging the file `Documentation/git-merge.txt`
+results in a conflict.
+
+------------------------
+Documentation/git-merge.txt	conflict-marker-size=32
+------------------------
+
+
 Checking whitespace errors
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 

t/t6026-merge-attr.sh

@@ -70,6 +70,18 @@ test_expect_success 'check merge result in working tree' '
 
 '
 
+test_expect_success 'retry the merge with longer context' '
+	echo text conflict-marker-size=32 >>.gitattributes &&
+	git checkout -m text &&
+	sed -ne "/^\([<=>]\)\1\1\1*/{
+		s/ .*$//
+		p
+	}" >actual text &&
+	grep ">>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>" actual &&
+	grep "================================" actual &&
+	grep "<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<" actual
+'
+
 cat >./custom-merge <<\EOF
 #!/bin/sh