.p4config root markers are useful to have

Looks like g:ctrlp_root_markers allows to add high priority root markers, but it doesn't work well if ctrlp is used from a path where somewhere up from current working directory both regular and high priority markers exist. Perforce (p4) is a bit of a special case, ~/.p4config is expected to be present for accessing p4 even without clients with source code.  And adding .p4config to g:ctrlp_root_markers will make ~/ root even when ctrlp is used from a git repo somewhere within one's home directory.  This simple fix should address the issue.