git-legacy-rebase: simplify unnecessary triply-nested if

git-legacy-rebase: simplify unnecessary triply-nested if

The git-legacy-rebase.sh script previously had code of the form:

if git_am_opt:
  if interactive:
    if incompatible_opts:
      show_error_about_interactive_and_am_incompatibilities
  if rebase-merge:
    if incompatible_opts
      show_error_about_merge_and_am_incompatibilities

which was a triply nested if.  However, the first conditional
(git_am_opt) and third (incompatible_opts) were somewhat redundant: the
latter condition was a strict subset of the former.  Simplify this by
moving the innermost conditional to the outside, allowing us to remove
the test on git_am_opt entirely and giving us the following form:

if incompatible_opts:
  if interactive:
    show_error_about_interactive_and_am_incompatibilities
  if rebase-merge:
    show_error_about_merge_and_am_incompatibilities

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>