parse-options: properly align continued usage output

Some commands such as "git stash" emit continued options output with
e.g. "git stash -h", because usage_with_options_internal() prefixes
with its own whitespace the resulting output wasn't properly
aligned. Let's account for the added whitespace, which properly aligns
the output.

The "git stash" command has usage output with a N_() translation that
legitimately stretches across multiple lines;

	N_("git stash [push [-p|--patch] [-k|--[no-]keep-index] [-q|--quiet]\n"
	   "          [-u|--include-untracked] [-a|--all] [-m|--message <message>]\n"
           [...]

We'd like to have that output aligned with the length of the initial
"git stash " output, but since usage_with_options_internal() adds its
own whitespace prefixing we fell short, before this change we'd emit:

    $ git stash -h
    usage: git stash list [<options>]
       or: git stash show [<options>] [<stash>]
       [...]
       or: git stash [push [-p|--patch] [-k|--[no-]keep-index] [-q|--quiet]
              [-u|--include-untracked] [-a|--all] [-m|--message <message>]
              [...]

Now we'll properly emit aligned output.  I.e. the last four lines
above will instead be (a whitespace-only change to the above):

       [...]
       or: git stash [push [-p|--patch] [-k|--[no-]keep-index] [-q|--quiet]
                     [-u|--include-untracked] [-a|--all] [-m|--message <message>]
                     [...]

We could also go for an approach where we have the caller support no
padding of their own, i.e. (same as the first example, except for the
padding on the second line):

	N_("git stash [push [-p|--patch] [-k|--[no-]keep-index] [-q|--quiet]\n"
	   "[-u|--include-untracked] [-a|--all] [-m|--message <message>]\n"
           [...]

But to do that we'll need to find the length of "git stash". We can
discover that from the "cmd" in the "struct cmd_struct", but there
might be cases with sub-commands or "git" itself taking arguments that
would make that non-trivial.

Even if it were I still think this approach is better, because this way
we'll get the same legible alignment in the C code. The fact that
usage_with_options_internal() is adding its own prefix padding is an
implementation detail that callers shouldn't need to worry about.

Implementation notes:

We could skip the string_list_split() with a strchr(str, '\n') check,
but we'd then need to duplicate our state machine for strings that do
and don't contain a "\n". It's simpler to just always split into a
"struct string_list", even though the common case is that that "struct
string_list" will contain only one element. This is not
performance-sensitive code.

This change is relatively more complex since I've accounted for making
it future-proof for RTL translation support. Later in
usage_with_options_internal() we have some existing padding code
dating back to d7a38c54a6c (parse-options: be able to generate usages
automatically, 2007-10-15) which isn't RTL-safe, but that code would
be easy to fix. Let's not introduce new RTL translation problems here.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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