tests/test-execute-bit

localrepo._tag: add a seek before writing the new tag

Some systems require a seek (or flush) between read and write
operations on the same file object.

This should fix issue838.

#!/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