patch 8.2.4792: indent operator creates an undo entry for every line

Problem:    Indent operator creates an undo entry for every line.
Solution:   Create one undo entry for all lines. (closes #10227)

src/indent.c

@@ -1060,10 +1060,14 @@ op_reindent(oparg_T *oap, int (*how)(void))
 	return;
     }
 
+    // Save for undo.  Do this once for all lines, much faster than doing this
+    // for each line separately, especially when undoing.
+    if (u_savecommon(start_lnum - 1, start_lnum + oap->line_count,
+				     start_lnum + oap->line_count, FALSE) == OK)
 	for (i = oap->line_count; --i >= 0 && !got_int; )
 	{
-	// it's a slow thing to do, so give feedback so there's no worry that
+	    // it's a slow thing to do, so give feedback so there's no worry
-	// the computer's just hung.
+	    // that the computer's just hung.
 
 	    if (i > 1
 		    && (i % 50 == 0 || i == oap->line_count - 1)
@@ -1083,7 +1087,7 @@ op_reindent(oparg_T *oap, int (*how)(void))
 		else
 		    amount = how();	    // get the indent for this line
 
-	    if (amount >= 0 && set_indent(amount, SIN_UNDO))
+		if (amount >= 0 && set_indent(amount, 0))
 		{
 		    // did change the indent, call changed_lines() later
 		    if (first_changed == 0)

src/testdir/test_indent.vim

@@ -59,6 +59,29 @@ func Test_reindent()
   close!
 endfunc
 
+" Test indent operator creating one undo entry
+func Test_indent_operator_undo()
+  enew
+  call setline(1, range(12)->map('"\t" .. v:val'))
+  func FoldExpr()
+    let g:foldcount += 1
+    return '='
+  endfunc
+  set foldmethod=expr foldexpr=FoldExpr()
+  let g:foldcount = 0
+  redraw
+  call assert_equal(12, g:foldcount)
+  normal gg=G
+  call assert_equal(24, g:foldcount)
+  undo
+  call assert_equal(38, g:foldcount)
+
+  bwipe!
+  set foldmethod& foldexpr=
+  delfunc FoldExpr
+  unlet g:foldcount
+endfunc
+
 " Test for shifting a line with a preprocessor directive ('#')
 func Test_preproc_indent()
   new

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4792,
 /**/
     4791,
 /**/