Fixes #3285 'defaultDateStyle' config being ignored by rebaseEditor

[TJohnsonSE]

Description

I fixed issue #3285 where the interactive rebase editor ignores the user's preference of showing "relative" or "absolute" date formats. The fix is a simple ternary evaluation of the config setting in rebaseEditor.ts.

This seemed like a good introductory issue to fix and get some exposure to the codebase. Gitlens is a wonderful extension that I use every day, so I just want to contribute what I can to a project that I find incredibly useful.

Checklist

• I have followed the guidelines in the Contributing document
• My changes follow the coding style of this project
• My changes build without any errors or warnings
• My changes have been formatted and linted
• My changes include any required corresponding changes to the documentation (including CHANGELOG.md and README.md)
• My changes have been rebased and squashed to the minimal number (typically 1) of relevant commits
• My changes have a descriptive commit message with a short title, including a Fixes $XXX - or Closes #XXX - prefix to auto-close the issue that your PR addresses