t6422: fix multiple errors with the mod6 test expectations

This test had multiple issues causing it to fail for the wrong
reason(s):

  * rename/rename(1to2) conflicts have always left the original source
    path present in the working directory and index (at stage 1).  Thus,
    the triple rename/rename(1to2) should result in 9 unstaged files,
    not 6.
  * It messed up the three-way content merge for checking the results of
    merging for one of the renames, accidentally turning it into a
    two-way merge.
  * It got the contents of the base files it was using to compare
    against wrong, due to an off-by-one error, and overwrite-redirection
    ('>') instead of append-redirection ('>>').
  * It used slightly too-long conflict markers
  * It didn't include filenames in the conflict marker hunks (granted,
    that was a shortcoming of the merge-recursive backend for rename/add
    and rename/rename(2to1) conflicts, but since it's
    test_expect_failure anyway we might as well make it expect our
    preferred behavior rather than some compromise that we can't yet
    reach anyway).

Fix these issues so that a merge backend which correctly handles these
kinds of nested conflicts will pass the test.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6422-merge-rename-corner-cases.sh

@@ -1042,25 +1042,25 @@ test_expect_failure 'mod6-check: chains of rename/rename(1to2) and rename/rename
 		test_must_be_empty err &&
 
 		git ls-files -s >file_count &&
-		test_line_count = 6 file_count &&
+		test_line_count = 9 file_count &&
 		git ls-files -u >file_count &&
-		test_line_count = 6 file_count &&
+		test_line_count = 9 file_count &&
 		git ls-files -o >file_count &&
 		test_line_count = 3 file_count &&
 
 		test_seq 10 20 >merged-one &&
 		test_seq 51 60 >merged-five &&
 		# Determine what the merge of three would give us.
-		test_seq 30 40 >three-side-A &&
+		test_seq 31 39 >three-base &&
+		test_seq 31 40 >three-side-A &&
 		test_seq 31 39 >three-side-B &&
-		echo forty >three-side-B &&
-		>empty &&
+		echo forty >>three-side-B &&
 		test_must_fail git merge-file \
-			-L "HEAD" \
+			-L "HEAD:four" \
 			-L "" \
-			-L "B^0" \
-			three-side-A empty three-side-B &&
-		sed -e "s/^\([<=>]\)/\1\1\1/" three-side-A >merged-three &&
+			-L "B^0:two" \
+			three-side-A three-base three-side-B &&
+		sed -e "s/^\([<=>]\)/\1\1/" three-side-A >merged-three &&
 
 		# Verify the index is as expected
 		git rev-parse >actual         \
@@ -1075,6 +1075,7 @@ test_expect_failure 'mod6-check: chains of rename/rename(1to2) and rename/rename
 
 		git cat-file -p :2:two >expect &&
 		git cat-file -p :3:two >other &&
+		>empty &&
 		test_must_fail git merge-file    \
 			-L "HEAD"  -L ""  -L "B^0" \
 			expect     empty  other &&