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>