[git] load commit message from .git/COMMIT_EDITMSG as well

Previously the commit message was loaded both from MERGE_MSG and
SQUASH_MSG.

* Load it from COMMIT_EDITMSG as well, but don't concatenate all
  messages, since in most cases they are the same: use only one of them.
* Extract loading message in a separate method.
[git] IDEA-64583 read commit message from .git if "amend" is selected

Listen to "Amend commit" checkbox state change:
- if user selects it, read the commit message from
  .git/COMMIT_EDITMSG (and other .git/*MSG if they exist)
- remember the commit message which was in the editor before user
  clicked "Amend" to restore it if he clicked accidentally.
- do it in a modal task to avoid UI freeze if reading from file is slow.
fix assertion