Obey p4 views when using client spec

Obey p4 views when using client spec

When using the p4 client spec, this attempts to obey the client's
output preferences.

For example, a view like

//depot/foo/branch/... //client/branch/foo/...
//depot/bar/branch/... //client/branch/bar/...

will result in a directory layout in the git tree of

branch/
branch/foo
branch/bar

p4 can do various other reordering that this change doesn't support,
but we should detect it and at least fail nicely.

Signed-off-by: Ian Wienand <ianw@vmware.com>
Acked-by: Pete Wyckoff <pw@padd.com>
Acked-by: Tor Arvid Lund <torarvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>