rebase -i: fix misleading error message after 'exec no-such' instruction

rebase -i: fix misleading error message after 'exec no-such' instruction

When the todo sheet of interactive rebase instructs to run a non-existing
command, the operation stops with the following error:

  Execution failed: no-such
  You can fix the problem, and then run

          git rebase --continue

  fatal: 'rebase' appears to be a git command, but we were not
  able to execute it. Maybe git-rebase is broken?

The reason is that the shell that attempted to run the command exits with
code 127. rebase--interactive just forwards this code to the caller (the
git wrapper). But our smart run-command infrastructure detects this
special exit code and turns it into ENOENT, which in turn is interpreted
by the git wrapper as if the external command that it just executed did
not exist. This is finally translated to the misleading last two lines in
error message cited above.

Fix it by translating the error code before it is forwarded.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>