patch 9.1.1615: diff format erroneously detected

Problem:  diff format erroneously detected
          (Tomáš Janoušek)
Solution: Make the regex to detect normal diff format a bit stricter,
          while at it, fix wrong test content from patch v9.1.1606

fixes: #17946

Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_filetype.vim

@@ -3027,7 +3027,7 @@ endfunc
 func Test_info_file()
   filetype on
 
-  call writefile(['File: coreutils.info,  Node: Top,  Next: Introduction,  Up: (dir)', 'D'], 'Xfile', 'D')
+  call writefile(['File: coreutils.info,  Node: Top,  Next: Introduction,  Up: (dir)'], 'Xfile', 'D')
   split Xfile
   call assert_equal('info', &filetype)
   bwipe!
@@ -3038,7 +3038,7 @@ endfunc
 func Test_mail_file()
   filetype on
 
-  call writefile(['Return-path: <lgc@debian.home.arpa>', 'D'], 'Xfile', 'D')
+  call writefile(['Return-path: <lgc@debian.home.arpa>'], 'Xfile', 'D')
   split Xfile
   call assert_equal('mail', &filetype)
   bwipe!
@@ -3049,7 +3049,7 @@ endfunc
 func Test_terminfo_file()
   filetype on
 
-  call writefile(['#	Reconstructed via infocmp from file: /etc/terminfo/x/xterm', 'D'], 'Xfile', 'D')
+  call writefile(['#	Reconstructed via infocmp from file: /etc/terminfo/x/xterm'], 'Xfile', 'D')
   split Xfile
   call assert_equal('terminfo', &filetype)
   bwipe!
@@ -3069,4 +3069,15 @@ func Test_pacmanlog()
   filetype off
 endfunc
 
+func Test_diff_format()
+  filetype on
+
+  call writefile(['0d555557  1 (John John   2025-01-01 00:00:00 +0000  1) foo'], 'Xdiff', 'D')
+  split Xdiff
+  call assert_true(empty(&filetype))
+  bwipe!
+
+  filetype off
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -719,6 +719,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1615,
 /**/
     1614,
 /**/