patch 8.2.4464: Dtrace files are recognized as filetype D

Problem:    Dtrace files are recognized as filetype D.
Solution:   Add a pattern for Dtrace files. (Teubel György, closes #9841)
            Add some more testing.
diff --git a/runtime/autoload/dist/ft.vim b/runtime/autoload/dist/ft.vim
index 4f5b88a..9f9fef0 100644
--- a/runtime/autoload/dist/ft.vim
+++ b/runtime/autoload/dist/ft.vim
@@ -206,6 +206,10 @@
 enddef
 
 export def DtraceCheck()
+  if did_filetype()
+    # Filetype was already detected
+    return
+  endif
   var lines = getline(1, min([line("$"), 100]))
   if match(lines, '^module\>\|^import\>') > -1
     # D files often start with a module and/or import statement.
diff --git a/runtime/filetype.vim b/runtime/filetype.vim
index 3b7f92d..77c736c 100644
--- a/runtime/filetype.vim
+++ b/runtime/filetype.vim
@@ -484,6 +484,7 @@
 au BufNewFile,BufRead *.desc			setf desc
 
 " the D language or dtrace
+au BufNewFile,BufRead */dtrace/*.d		setf dtrace
 au BufNewFile,BufRead *.d			call dist#ft#DtraceCheck()
 
 " Desktop files