commit: disable status hints when writing to COMMIT_EDITMSG

commit: disable status hints when writing to COMMIT_EDITMSG

This turns the template COMMIT_EDITMSG from e.g

  # [...]
  # Changes to be committed:
  #   (use "git reset HEAD <file>..." to unstage)
  #
  #	modified:   builtin/commit.c
  #
  # Untracked files:
  #   (use "git add <file>..." to include in what will be committed)
  #
  #	t/foo
  #

to

  # [...]
  # Changes to be committed:
  #	modified:   builtin/commit.c
  #
  # Untracked files:
  #	t/foo
  #

Most status hints were written to be accurate when running "git status"
before running a commit. Many of them are not applicable when the commit
has already been started, and should not be shown in COMMIT_EDITMSG. The
most obvious are hints advising to run "git commit",
"git rebase/am/cherry-pick --continue", which do not make sense when the
command has already been run.

Other messages become slightly inaccurate (e.g. hint to use "git add" to
add untracked files), as the suggested commands are not immediately
applicable during the editing of COMMIT_EDITMSG, but would be applicable
if the commit is aborted. These messages are both potentially helpful and
slightly misleading. This patch chose to remove them too, to avoid
introducing too much complexity in the status code.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>