patch: fix clash between local variable and exception instance

The local err variable would be bound to PatchError thrown and it
keeps this value even after the except block is executed.

The whole thing worked anyway since the rejected variable is set in
the except block and this makes the function return -1 when a
PatchError is thrown.

mercurial/patch.py

@@ -1132,8 +1132,8 b' def _applydiff(ui, fp, patcher, copyfn, '
                                                    first_hunk, strip)
                 current_file = patcher(ui, current_file, opener,
                                        missing=missing, eolmode=eolmode)
-            except PatchError, err:
-                ui.warn(str(err) + '\n')
+            except PatchError, inst:
+                ui.warn(str(inst) + '\n')
                 current_file = None
                 rejects += 1
                 continue