mergetool: honor diff.orderFile

Teach mergetool to get the list of files to edit via `diff` so that we
gain support for diff.orderFile.

Suggested-by: Luis Gutierrez <luisgutz@gmail.com>
Helped-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: David Aguilar <davvid@gmail.com>
Reviewed-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-mergetool.txt

@@ -79,6 +79,11 @@ success of the resolution after the custom tool has exited.
 	Prompt before each invocation of the merge resolution program
 	to give the user a chance to skip the path.
 
+DIFF ORDER FILES
+----------------
+`git mergetool` honors the `diff.orderFile` configuration variable
+used by `git diff`.  See linkgit:git-config[1] for more details.
+
 TEMPORARY FILES
 ---------------
 `git mergetool` creates `*.orig` backup files while resolving merges.

git-mergetool.sh

@@ -382,6 +382,11 @@ prompt_after_failed_merge () {
 	done
 }
 
+print_noop_and_exit () {
+	echo "No files need merging"
+	exit 0
+}
+
 main () {
 	prompt=$(git config --bool mergetool.prompt)
 	guessed_merge_tool=false
@@ -445,28 +450,23 @@ main () {
 	merge_keep_backup="$(git config --bool mergetool.keepBackup || echo true)"
 	merge_keep_temporaries="$(git config --bool mergetool.keepTemporaries || echo false)"
 
-	files=
-
+	if test $# -eq 0 && test -e "$GIT_DIR/MERGE_RR"
+	then
+		set -- $(git rerere remaining)
 		if test $# -eq 0
 		then
-		cd_to_toplevel
+			print_noop_and_exit
+		fi
+	fi
 
-		if test -e "$GIT_DIR/MERGE_RR"
-		then
-			files=$(git rerere remaining)
-		else
-			files=$(git ls-files -u |
-				sed -e 's/^[^	]*	//' | sort -u)
-		fi
-	else
-		files=$(git ls-files -u -- "$@" |
-			sed -e 's/^[^	]*	//' | sort -u)
-	fi
+	files=$(git -c core.quotePath=false \
+		diff --name-only --diff-filter=U -- "$@")
+
+	cd_to_toplevel
 
 	if test -z "$files"
 	then
-		echo "No files need merging"
-		exit 0
+		print_noop_and_exit
 	fi
 
 	printf "Merging:\n"

t/t7610-mergetool.sh

@@ -606,4 +606,37 @@ test_expect_success MKTEMP 'temporary filenames are used with mergetool.writeToT
 	git reset --hard master >/dev/null 2>&1
 '
 
+test_expect_success 'diff.orderFile configuration is honored' '
+	test_config diff.orderFile order-file &&
+	test_config mergetool.myecho.cmd "echo \"\$LOCAL\"" &&
+	test_config mergetool.myecho.trustExitCode true &&
+	echo b >order-file &&
+	echo a >>order-file &&
+	git checkout -b order-file-start master &&
+	echo start >a &&
+	echo start >b &&
+	git add a b &&
+	git commit -m start &&
+	git checkout -b order-file-side1 order-file-start &&
+	echo side1 >a &&
+	echo side1 >b &&
+	git add a b &&
+	git commit -m side1 &&
+	git checkout -b order-file-side2 order-file-start &&
+	echo side2 >a &&
+	echo side2 >b &&
+	git add a b &&
+	git commit -m side2 &&
+	test_must_fail git merge order-file-side1 &&
+	cat >expect <<-\EOF &&
+		Merging:
+		b
+		a
+	EOF
+	git mergetool --no-prompt --tool myecho >output &&
+	git grep --no-index -h -A2 Merging: output >actual &&
+	test_cmp expect actual &&
+	git reset --hard >/dev/null
+'
+
 test_done