[sheets-] speed up loading check of max_rows

For a sheet with millions of rows, the inner loop max_rows check was
delaying loading time by 60%.

visidata/sheets.py

@@ -303,8 +303,9 @@ def loader(self):
         self.rows = []
         try:
             with vd.Progress(gerund='loading', total=0):
+                m = self.options.max_rows
                 for i, r in enumerate(self.iterload()):
-                    if self.precious and i > self.options.max_rows:
+                    if self.precious and i == m:
                         break
                     self.addRow(r)
         except FileNotFoundError:
@@ -1051,8 +1052,9 @@ def loader(self):
 
         self.rows = []
         # add the rest of the rows
+        m = self.options.max_rows
         for i, r in enumerate(vd.Progress(itsource, gerund='loading', total=0)):
-            if self.precious and i > self.options.max_rows:
+            if self.precious and i == m:
                 break
             self.addRow(r)