patch 8.2.2880: unified diff fails if actually used

Problem:    Unified diff fails if actually used.
Solution:   Invoke :diffupdate in the test.  Fix the check for working external
            diff. (Ghjuvan Lacambre, Christian Brabandt, closes #8197)

src/diff.c

@@ -1007,10 +1007,12 @@ check_external_diff(diffio_T *diffio)
 
 		    for (;;)
 		    {
-			// There must be a line that contains "1c1".
+			// For normal diff there must be a line that contains
+			// "1c1".  For unified diff "@@ -1 +1 @@".
 			if (vim_fgets(linebuf, LBUFLEN, fd))
 			    break;
-			if (STRNCMP(linebuf, "1c1", 3) == 0)
+			if (STRNCMP(linebuf, "1c1", 3) == 0
+				|| STRNCMP(linebuf, "@@ -1 +1 @@", 11) == 0)
 			    ok = TRUE;
 		    }
 		    fclose(fd);

src/testdir/test_diffmode.vim

@@ -796,6 +796,7 @@ func VerifyBoth(buf, dumpfile, extra)
 
   " also test unified diff
   call term_sendkeys(a:buf, ":call SetupUnified()\<CR>:")
+  call term_sendkeys(a:buf, ":redraw!\<CR>:")
   call VerifyScreenDump(a:buf, a:dumpfile, {}, 'unified')
   call term_sendkeys(a:buf, ":call StopUnified()\<CR>:")
 endfunc
@@ -817,10 +818,11 @@ func Test_diff_screen()
       func UnifiedDiffExpr()
         " Prepend some text to check diff type detection
         call writefile(['warning', '  message'], v:fname_out)
-        silent exe '!diff -u ' .. v:fname_in .. ' ' .. v:fname_new .. '>>' .. v:fname_out
+        silent exe '!diff -U0 ' .. v:fname_in .. ' ' .. v:fname_new .. '>>' .. v:fname_out
       endfunc
       func SetupUnified()
         set diffexpr=UnifiedDiffExpr()
+        diffupdate
       endfunc
       func StopUnified()
         set diffexpr=
@@ -1218,4 +1220,5 @@ func Test_diff_filler_cursorcolumn()
   call delete('Xtest_diff_cuc')
 endfunc
 
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2880,
 /**/
     2879,
 /**/