[git] Save amended message to restore it if edited by user

If user selects "Amend commit", edits it and then accidentally
deselects the checkbox, the content after his edits is lost.
Preserve it, and restore if he returns to "amended".
This also lets avoid requesting Git several times for each
click to "Amend".

* Extract loading from Git in a modal task to a separate method.
* When the checkbox is deselected, remember commit area text to
myAmendedMessage.
* Restore it when the checkbox is selected again.
wrong background
customize radio button painting
radio buttons colors
better colors
IDEA-114982 (Feature: warn if something is calculated inside assert statement)
fix check box colors