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
Signed-off-by: Orgad Shaneh <orgad.shaneh@audiocodes.com>