cat-file: correct and improve usage information

cat-file: correct and improve usage information

Change the usage output emitted on "git cat-file -h" to group related
options, making it clear to users which options go with which other
ones.

The new output is:

    Check object existence or emit object contents
        -e                    check if <object> exists
        -p                    pretty-print <object> content

    Emit [broken] object attributes
        -t                    show object type (one of 'blob', 'tree', 'commit', 'tag', ...)
        -s                    show object size
        --allow-unknown-type  allow -s and -t to work with broken/corrupt objects

    Batch objects requested on stdin (or --batch-all-objects)
        --batch[=<format>]    show full <object> or <rev> contents
        --batch-check[=<format>]
                              like --batch, but don't emit <contents>
        --batch-all-objects   with --batch[-check]: ignores stdin, batches all known objects

    Change or optimize batch output
        --buffer              buffer --batch output
        --follow-symlinks     follow in-tree symlinks
        --unordered           do not order objects before emitting them

    Emit object (blob or tree) with conversion or filter (stand-alone, or with batch)
        --textconv            run textconv on object's content
        --filters             run filters on object's content
        --path blob|tree      use a <path> for (--textconv | --filters ); Not with 'batch'

The old usage was:

    <type> can be one of: blob, tree, commit, tag
        -t                    show object type
        -s                    show object size
        -e                    exit with zero when there's no error
        -p                    pretty-print object's content
        --textconv            for blob objects, run textconv on object's content
        --filters             for blob objects, run filters on object's content
        --batch-all-objects   show all objects with --batch or --batch-check
        --path <blob>         use a specific path for --textconv/--filters
        --allow-unknown-type  allow -s and -t to work with broken/corrupt objects
        --buffer              buffer --batch output
        --batch[=<format>]    show info and content of objects fed from the standard input
        --batch-check[=<format>]
                              show info about objects fed from the standard input
        --follow-symlinks     follow in-tree symlinks (used with --batch or --batch-check)
        --unordered           do not order --batch-all-objects output

While shorter, I think the new one is easier to understand, as
e.g. "--allow-unknown-type" is grouped with "-t" and "-s", as it can
only be combined with those options. The same goes for "--buffer",
"--unordered" etc.

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