Change the perforce polling command to use p4 changes -m2

Raised from BSP-1112 -

I am facing major performance drawbacks on perforce server resulting from bamboo polling. The fact that bamboo uses 'p4 changes -m 1' to check for changes affects perforce performance. This is noted on large depots.
We faced this problem with perforce earlier and got in touch with perforce support. It turned out to be due to the way p4 server interprets this commands and precesses it. The proposed workaround was to use 'p4 changes -m 2' and parse the 1st output line only.