[PATCH 2 of 2] repovfs: add a ward to check if locks are properly taken

[timeless]

Pierre-Yves David wrote:
> We use the 'ward' feature introduced in the previous email to check for locks

`email` is wrong, sure I'm reading it in an email, but everyone else
is seeing it in a Commit message in the HG log...

> when accessing file in '.hg' for writting. Another changeset will add a 'ward'

a file | files

> This 'ward' system have caught an handful of locking issue that  has been fixed

has caught a handful ... that have been