Add --reset-author flag

[spalger]

When amending a commit you can use the --reset-author flag to redefine the author of the commits as yourself. This is generally not recommended, but is desired for backporting automated commits like those done by renovate (mostly so that CLA checking works).

[spalger]

Example of doing this manually: elastic/kibana#44000

git co -b backport/7.x/pr-{n}
git cherry-pick -xn {sha}
git commit --author "{username} <{username}@users.noreply.github.com>"
# prefix pr title with [{branch}]
git push origin ...