getClientHead without range

[kjs104901]

Our Perforce server has been operational since July 2, 2013, and the current changelist number is 1524832.

Normally, using changes -m is efficient for both users and build machines due to our optimization settings.
(For more details, see Server performance related to 'p4 changes').

However, retrieving changes for a specific client and revision is significantly slower
(changes -m1 //clientname/...@Rev).
This slowdown affects the plugin's ability to determine the latest changelist number for clients.

I believe that setting a revision range for changes is not essential, and omitting it could enhance performance.
According to the documentation, optimizations are most effective when not restricted by revision specifications.

I have already implemented this change in our Jenkins setup, and it has shown positive results.

Before

After

Testing done

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[skumar7322]

Thanks @kjs104901 for creating this PR to improve P4-Plugin. Will take up this in our next release.