Minor Compiler Warning

[aksuska]

GCUndoManager.m, -perform
Implicit signedness conversion at lines 1331 and 1334--seems better to just declare "i" as NSUInteger, as that seems consistent with use.

[seanm]

Care to create a pull request? Also, be sure that the 'while' loop does not need reformulating... 0-1 is different with signed vs unsigned math...

[aksuska]

The while loop is OK I think--I added some discussion of overflow issues in the pull request. It all depends on how likely the method will be invoked on an empty group.