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 % should fail
hg add .hg/00changelog.i
mkdir a
echo a > a/a
hg ci -Ama
ln -s a b
echo b > a/b
echo % should fail
hg add b/b
echo % should succeed
hg add b
echo % should still fail - maybe
hg add b/b
exit 0