git-gui: Do not reset author details on amend

[orgads]

git commit --amend preserves the author details unless --reset-author is
given.

git-gui discards the author details on amend.

Fix by reading the author details along with the commit message, and
setting the appropriate environment variables required for preserving
them.

Reported long ago in the mailing list[1].

[1] http://article.gmane.org/gmane.comp.version-control.git/243921

[patthoyts]

Note for me: check to see if this is responsible for the problem at http://stackoverflow.com/q/39724766/291641

[orgads]

It is, and this is by design. git commit --amend preserves author time. I answered your question there.

[patthoyts]

As pointed out by the OP for the stackoverflow question, the method used here is excessively sticky. Currently this has only been applied to the git for windows version but needs resolving before it gets merged upstream here.

[patthoyts]

@orgads: I've added another change to branch os/preserve-author which stops this change to the author details from persisting for the rest of the session and reworked the changes to avoid adding more globals. If you could check that it's still working for you then I can merge the two commits into git-gui's master.
As your original is merged to git-for-windows already I've left it as is and done all changes as a new commit on top. ( https://github.com/patthoyts/git-gui/tree/os/preserve-author )