Merge branch 'git-p4' of git://repo.or.cz/git/git-p4

* 'git-p4' of git://repo.or.cz/git/git-p4: git-p4: Use P4EDITOR environment variable when set git-p4: Unset P4DIFF environment variable when using 'p4 -du diff' git-p4: Optimize the fetching of data from perforce.