fast-export: standardize usage string and SYNOPSIS

fast-export: standardize usage string and SYNOPSIS

The output of `git fast-export -h` currently starts with:

  usage: git fast-export [<rev-list-opts>]

while the SYNOPSIS section in this command's documentation shows:

  'git fast-export' [<options>] | 'git fast-import'

Let's make both of these consistent with each other and with other Git
commands by describing the arguments with:

  [<options>] [<revision-range>] [[--] <path>...]

This takes into account the following:

  - `git fast-export` accepts both rev-list arguments and a number of
    genuine options of its own (--[no-]progress, --[no-]signed-tags,
    --[no-]signed-commits, etc).

  - `git fast-export` was the only command using `[<rev-list-opts>]`
    while many other commands describe their revision arguments as
    `[<revision-range>] [[--] <path>...]`.

  - In the DESCRIPTION section of the documentation, it's already
    mentioned several times that the output should eventually be fed to
    `git fast-import`.

This also enables us to remove fast-export from
"t/t0450/adoc-help-mismatches".

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>