fast-export, fast-import: add support for signed-commits

This is to address: #457 (closed)

Luke Shumaker sent the first 4 versions of this series in April 2021, but it looks like he stopped before it got merged. Let's finish polishing it.

Goal:

fast-export has an existing --signed-tags= option that controls how to handle tag signatures. However, there is no equivalent for commit signatures; it just silently strips the signature out of the commit (analogously to --signed-tags=strip).

So implement a --signed-commits= flag in fast-export, and implement the receiving side of it in fast-import.