See if execute bit is honoured when we go back in time

tests/test-execute-bit

@@ -0,0 +1,13 b''
+#!/bin/sh
+
+hg init
+echo a > a
+hg ci -d'0 0' -Am'not executable'
+
+chmod +x a
+hg ci -d'1 0' -m'executable'
+hg id
+
+hg up 0
+hg id
+test -x a && echo executable -- eek || echo not executable -- whew

tests/test-execute-bit.out

@@ -0,0 +1,5 b''
+adding a
+1549299e88d1 tip
+0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+d69afc33ff8a
+not executable -- whew