Allow amending commit content, not only the message

[vais]

Short description of the problem

Currently, amending a commit in vimagit does not load/show/allow manipulating the list of files that were committed, only the commit message.

Compare this with e.g. the native git gui command: when you choose to amend last commit, it displays as staged the list of files that were added/changed/removed by the commit, and allows user to un-stage these "staged" files from the commit, stage other files, etc. This is a very useful feature to have when amending a commit.

I imagine this working in vimagit the same way regular commit mode works right now, except the staged file list would be pre-loaded from the commit being amended, the same way the commit message is currently pre-loaded.

Hope this makes sense. Thanks!