git-p4: don't groom exclude path list on every commit

git-p4: don't groom exclude path list on every commit

Currently, `cloneExclude` array is being groomed (by removing trailing "...")
on every changeset.
(since `extractFilesFromCommit()` is called on every imported changeset)

As a micro-optimization, do it once while parsing arguments.
Also, prepend "/" and remove trailing "..." at the same time.

Signed-off-by: Andrey Mazo <amazo@checkvideo.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>