Replace deprecated NSOnState/NSOffState with NSControlStateValue constants - #536

Eliminates macOS 10.14+ deprecation warnings by migrating from NSOnState/NSOffState to NSControlStateValueOn/NSControlStateValueOff.

Changes

• Menu item states: Updated state assignments in history controller (detail/tree view), commit controller (amend toggle), and search controller (search mode selection)
• Checkbox/button state checks: Updated state comparisons in sheets and panels for hidden file toggles and alert suppression buttons

Example

// Before
[menuItem setState:(condition) ? NSOnState : NSOffState];
if ([sender state] == NSOnState) { ... }

// After  
[menuItem setState:(condition) ? NSControlStateValueOn : NSControlStateValueOff];
if ([sender state] == NSControlStateValueOn) { ... }

Affects 8 occurrences across 7 files: PBGitHistoryController.m, PBGitCommitController.m, PBHistorySearchController.m, PBPrefsWindowController.m, PBGitWindowController.m, PBAddRemoteSheet.m, PBCloneRepositoryPanel.m.

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