Add `--amend` option

[lucatrv]

Is your feature request related to a problem? Please describe.
Presently koji can add new commits, but it cannot amend the last commit.

Describe the solution you'd like
koji --amend should amend the last commit.

Additional context
Each question should be pre-filled with the information coming from the last commit.

[cococonscious]

Since koji currently relies on cocogitto to commit, it needs to be implemented there first. Alternatively we could also move away from cocogitto for committing, it's missing a few features koji wants anyways (e.g. submodule support), but this needs a thorough evaluation first.