Uses VSCode as core.editor for rebase

[ShafinKhadem]

Description

When using git commands from terminal, one may prefer to use terminal editors to edit commit messages. But when using VSCode, it's better to use VSCode to edit commit messages. Currently gitlens uses VSCode to edit commit messages for amend. However it uses git's global core.editor for 'reword' in rebase. This PR ensures the usage of VSCode as core.editor for rebase.

Closes #2084

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
• 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

[eamodio]

Thank you so much for your contribution!