git-p4: recover from inconsistent perforce history

git-p4: recover from inconsistent perforce history

Perforce allows you commit files and directories with the same name,
so you could have files //depot/foo and //depot/foo/bar both checked
in.  A p4 sync of a repository in this state fails.  Deleting one of
the files recovers the repository.

When this happens we want git-p4 to recover in the same way as
perforce.

Note that Perforce has this change in their 2017.1 version:

     Bugs fixed in 2017.1
     #1489051 (Job #2170) **
        Submitting a file with the same name as an existing depot
        directory path (or vice versa) will now be rejected.

so people hopefully will not creating damaged Perforce repos
anymore, but "git p4" needs to be able to interact with already
corrupt ones.

Signed-off-by: Andrew Oakley <andrew@adoakley.name>
Reviewed-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>