Fix virtual graph lines claiming shared history

[Shriram-Vasudevan]

Fixes #336.

Summary

• process real graph nodes before dummy/virtual branch-tip nodes when generating each layer
• prevents a virtual branch tip from becoming the primary line for a shared commit and rendering the rest of its ancestry as thin/dashed
• add a regression test with out-of-order committer dates, matching the rebase/amend scenario described in the issue

Verification

• clang -fsyntax-only ... GitUpKit/Interface/GIGraph.m
• git diff --no-index --check GitUp-baseline GitUp

Could not run the XCTest suite locally because this machine only has Command Line Tools installed; xcodebuild requires full Xcode. clang-format is also unavailable locally.