When using P4CONFIG one can user multiple p4 workspaces.

For every workspace one needs to have a p4config.txt file,
which contains the P4CLIENT, P4USER, P4PORT variable values.

Qt Creator will query on various directories for p4 configuration.
"p4 client" command will give an error if a p4config.txt file is
not present. p4 will try to connect to localhost when P4PORT is
not set and give and error, but it will take a long time to
timeout.

This timeout gives the impression that P4 does a lot of work and
Qt Creator will sometimes freeze.

The solution to this timeout is to set P4PORT and P4USER. In
this case "p4 client" will give back the same generic "Root:"
variable e.g. c:\p4, which will be seen as a valid path and
PerforcePlugin::setTopLevel will be called and thus setting the
path of the current workspace, which is not valid.

Fortunately in this case "Client:" and "Host:" variables in the
output given by the "p4 client" have the same value. "Client:"
will probably never have the same value as the "Host:" value,
thus making it safe to ignore these invalid "p4 client" calls.

Change-Id: Iafb2640cccd7bd10c455d33ea9d36c5eae2e2951
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>