amend: use "getcommiteditor()" instead of explicit editor choice This patch doesn't change any tests like as preceding patches, because editor invocation is already tested in "test-commit-amend.t".

commit: use "getcommiteditor()" instead of explicit editor choice This patch also eliminates "forceeditor" no more referred. This patch doesn't change any tests like as preceding patches, because editor invocation is already tested well.