Write .editmsg in GIT_DIR to avoid being in git-status.

Write .editmsg in GIT_DIR to avoid being in git-status.

It appears in the git-status output during a git-commit if you have
something in info/exclude.

Also for .cmitmsg and .cmitchk to make git-commit work
in read-only working trees.

[jc: while we are at it, I removed the use of .cmitchk temporary
     file which was not necessary, and renamed them -- they are out
     of way now and do not have to be dotfiles anymore.]

Signed-off-by: Santi Béjar <sbejar@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>