Codify changes to support versioning of P4DTG common/dtg folder.
Currently, we avoid versioning the /p4/common/dtg folder in the SDP
structure where P4DTG runs, because P4DTG puts a mix of static
configuration files (which should be versioned) and dynamic files
including certain XML and log files (which should not be versioned)
in the same directory.
To version that folder would require a versioned P4IGNORE file
that appears as /p4/common/dtg/.p4ignore. Then the default
SDP_STATUS_DIRS setting in sdp_hosts.cfg could be expanded to
include the /p4/common/dtg folder.
See also: SDP-700 and P4DTG job job108521 (Perforce-internal).