Fix yet another bug with diff file names (#12771) (#12776)

Backport #12771

Following further testing it has become apparent that the diff line
cannot be used to determine filenames for diffs with any sort of predictability
the answer therefore is to use the other lines that are provided with a diff

Fix #12768

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/templates/helper.go

@@ -619,7 +619,7 @@ func ActionContent2Commits(act Actioner) *repository.PushCommits {
 // DiffTypeToStr returns diff type name
 func DiffTypeToStr(diffType int) string {
 	diffTypes := map[int]string{
-		1: "add", 2: "modify", 3: "del", 4: "rename",
+		1: "add", 2: "modify", 3: "del", 4: "rename", 5: "copy",
 	}
 	return diffTypes[diffType]
 }