git rebase -i: warn about removed commits

git rebase -i: warn about removed commits

Check if commits were removed (i.e. a line was deleted) and print
warnings or stop git rebase depending on the value of the
configuration variable rebase.missingCommitsCheck.

This patch gives the user the possibility to avoid silent loss of
information (losing a commit through deleting the line in this case)
if he wants.

Add the configuration variable rebase.missingCommitsCheck.
    - When unset or set to "ignore", no checking is done.
    - When set to "warn", the commits are checked, warnings are
      displayed but git rebase still proceeds.
    - When set to "error", the commits are checked, warnings are
      displayed and the rebase is stopped.
      (The user can then use 'git rebase --edit-todo' and
      'git rebase --continue', or 'git rebase --abort')

rebase.missingCommitsCheck defaults to "ignore".

Signed-off-by: Galan Rémi <remi.galan-alfonso@ensimag.grenoble-inp.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>