patch 8.2.3843: dep3patch files are not recognized

Problem:    Dep3patch files are not recognized.
Solution:   Recognize dep3patch files by their location and content. (James
            McCoy, closes #9367)
diff --git a/src/testdir/test_filetype.vim b/src/testdir/test_filetype.vim
index f4204f7..cd82e38 100644
--- a/src/testdir/test_filetype.vim
+++ b/src/testdir/test_filetype.vim
@@ -1029,4 +1029,44 @@
   filetype off
 endfunc
 
+func Test_dep3patch_file()
+  filetype on
+
+  call assert_true(mkdir('debian/patches', 'p'))
+
+  " series files are not patches
+  call writefile(['Description: some awesome patch'], 'debian/patches/series')
+  split debian/patches/series
+  call assert_notequal('dep3patch', &filetype)
+  bwipe!
+
+  " diff/patch files without the right headers should still show up as ft=diff
+  call writefile([], 'debian/patches/foo.diff')
+  split debian/patches/foo.diff
+  call assert_equal('diff', &filetype)
+  bwipe!
+
+  " Files with the right headers are detected as dep3patch, even if they don't
+  " have a diff/patch extension
+  call writefile(['Subject: dep3patches'], 'debian/patches/bar')
+  split debian/patches/bar
+  call assert_equal('dep3patch', &filetype)
+  bwipe!
+
+  " Files in sub-directories are detected
+  call assert_true(mkdir('debian/patches/s390x', 'p'))
+  call writefile(['Subject: dep3patches'], 'debian/patches/s390x/bar')
+  split debian/patches/s390x/bar
+  call assert_equal('dep3patch', &filetype)
+  bwipe!
+
+  " The detection stops when seeing the "header end" marker
+  call writefile(['---', 'Origin: the cloud'], 'debian/patches/baz')
+  split debian/patches/baz
+  call assert_notequal('dep3patch', &filetype)
+  bwipe!
+
+  call delete('debian/patches', 'rf')
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab
diff --git a/src/version.c b/src/version.c
index d90560f..0e2a229 100644
--- a/src/version.c
+++ b/src/version.c
@@ -750,6 +750,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    3843,
+/**/
     3842,
 /**/
     3841,