Execution bit detection fixes for VFAT on Linux

On Linux VFAT execution mode can be modified, but changes don't
persist a filesy stem remount. The current test can be trickled by
this. We can help with the det ection of VFAT checking whether new
files get created with the execution bits on
 (as usually these partitions are mounted with the exec option, for
convenience)
.

mercurial/util.py

@@ -851,16 +851,20 b' def checkexec(path):'
     Requires a directory (like /foo/.hg)
     """
     try:
+        EXECFLAGS = stat.S_IXUSR | stat.S_IXGRP | stat.S_IXOTH
         fh, fn = tempfile.mkstemp("", "", path)
         os.close(fh)
         m = os.stat(fn).st_mode
-        os.chmod(fn, m ^ 0111)
-        r = (os.stat(fn).st_mode != m)
+        # VFAT on Linux can flip mode but it doesn't persist a FS remount.
+        # frequently we can detect it if files are created with exec bit on.
+        new_file_has_exec = m & EXECFLAGS
+        os.chmod(fn, m ^ EXECFLAGS)
+        exec_flags_cannot_flip = (os.stat(fn).st_mode == m)
         os.unlink(fn)
     except (IOError,OSError):
         # we don't care, the user probably won't be able to commit anyway
         return False
-    return r
+    return not (new_file_has_exec or exec_flags_cannot_flip)
 
 def execfunc(path, fallback):
     '''return an is_exec() function with default to fallback'''