Relying on the exact return of statwalk would cause us to abort
when there was at least one tracked file inside an ignored directory.
This patch forces an extra walk of the whole working directory even
on sane filesystems, where it wouldn't be needed.
Fixes issue621.
In a name mangling filesystem (e.g. a case insensitive one)
dirstate.walk() can yield filenames different from the ones
stored in the dirstate. This already confuses the status and
add commands, but with purge this may cause data loss.
To prevent this purge refuses to work if there are missing
files and has a 'force' option if the user knows it is safe.
Even with the force option purge checks if any of the missing
files is still available in the working dir: if so there
may be some problem with the underlying filesystem, so it
unconditionally aborts.
