tests: assert consistent whitespace in -h output

tests: assert consistent whitespace in -h output

Add a test for the *.txt and *.c output assertions which asserts that
for "-h" lines that aren't the "usage: " or " or: " lines they start
with the same amount of whitespace. This ensures that we won't have
buggy output like:

   [...]
   or: git tag [-n[<num>]]
               [...]
       [--create-reflog] [...]

Which should instead be like this, i.e. the options lines should be
aligned:

   [...]
   or: git tag [-n[<num>]]
               [...]
               [--create-reflog] [...]

It would be better to be able to use "test_cmp" here, i.e. to
construct the output we expect, and compare it against the actual
output.

For most built-in commands this would be rather straightforward. In
"t0450-txt-doc-vs-help.sh" we already compute the whitespace that a
"git-$builtin" needs, and strip away "usage: " or " or: " from the
start of lines. The problem is:

 * For commands that implement subcommands, such as "git bundle", we
   don't know whether e.g. "git bundle create" is the subcommand
   "create", or the argument "create" to "bundle" for the purposes of
   alignment.

   We *do* have that information from the *.txt version, since the
   part within the ''-quotes should be the command & subcommand, but
   that isn't consistent (e.g. see "git bundle" and "git
   commit-graph", only the latter is correct), and parsing that out
   would be non-trivial.

 * If we were to make this stricter we have various
   non-parse_options() users (e.g. "git diff-tree") that don't have the
   nicely aligned output which we've had since
   4631cfc20bd (parse-options: properly align continued usage output,
   2021-09-21).

So rather than make perfect the enemy of the good let's assert that
for those lines that are indented they should all use the same
indentation.

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