Kill the buffer used for the empty diff check if no diffs were found.
Use save-some-buffers instead of p4-save-opened-files.
(The former has more features, and users will be used it since the compile command already runs it.)
p4-depot-find-file now runs "p4 where" itself (instead of via p4-map-depot-files) so that it can report the error from Perforce if the file neither exists on the client or on the server.
Removed unused functions p4-get-client-root, p4-canonize-client-root, p4-map-depot-files, p4-map-depot-files-int, p4-save-opened-files.
Remove unused user option p4-exec-arg-len-max.
Fixes #9.