submodule: fix fetch_in_submodule logic

submodule: fix fetch_in_submodule logic

Commit 1c1518071c (submodule: use "fetch" logic instead of custom remote
discovery, 2020-11-14) rewrote the logic in fetch_in_submodule to do:

  elif test "$2" -ne ""

But this is nonsense in shell: -ne is for numeric comparisons. This
should be "=" or more idiomatically:

  elif test -n "$2"

But once we fix that, many tests start failing. Because that commit
introduced another problem. The caller that passes 3 arguments looks
like this:

    fetch_in_submodule "$sm_path" $depth "$sha1"

Note the unquoted $depth parameter. When it isn't set, the function will
see only 2 arguments, and the function has no idea if what it sees in $2
is an option to go on the command line, or a refspec to pass on stdin.
In the old code before that commit:

   fetch_in_submodule () (
        sanitize_submodule_env &&
        cd "$1" &&
  -     case "$2" in
  -     '')
  -             git fetch ;;
  -     *)
  -             shift
  -             git fetch $(get_default_remote) "$@" ;;
  -     esac

we treated those the same, so it didn't matter. But in the new logic
(with my fix above):

  +     if test $# -eq 3
  +     then
  +             echo "$3" | git fetch --stdin "$2"
  +     elif test -n "$n"
  +     then
  +             git fetch "$2"
  +     else
  +             git fetch
  +     fi

we use the number of parameters to distinguish the two. Let's insist
that the caller pass an empty string for positional parameter two if
they want to have a third parameter after it.

But that still leaves one problem. In the --stdin block, we
unconditionally pass "$2" to git-fetch, even if it's the empty string.
Rather than add another conditional, we can use :+ parameter expansion
to include it only if it's non-empty. In fact, we can do the same for
the elif, too, simplifying it further. Technically this is overkill,
since we know the --depth parameter will not have whitespace (and
indeed, most callers do not bother quoting it), but it doesn't hurt for
the function to be careful.

It's somewhat amazing that no tests were failing. I think what happened
is that:

  - the 3-arg form rarely triggered; any call with a non-empty $depth
    and a $sha1 would work, but one with an empty $depth would only have
    2 arguments

  - because of the wrong arguments to "test", the shell would complain
    and exit non-zero. So we never ran the middle conditional at all

  - that left every call running "git fetch" with no arguments. A
    well-written test could have detected the distinction here, but in
    practice omitting --depth just means fetching more commits, and
    fetching everything (rather than a single sha1) works as long as the
    commit in question is reachable

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>