Uses actual exec path as editor command

[nzaytsev]

Description

The interactive rebase is a native git feature that uses code or another similar target as an editor. If the editor is portable or there is no env path to the editor for some other reason, then git can not open it. To fix it we can use direct app path that can be resolved from vscode.env.appRoot and some minor redirection to bin/code.

For windows and linux it's solved by ../../bin/code
For MacOS - ./bin/code

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