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>

Documentation/git-fast-export.adoc

@@ -9,7 +9,7 @@ git-fast-export - Git data exporter
 SYNOPSIS
 --------
 [verse]
-'git fast-export' [<options>] | 'git fast-import'
+'git fast-export' [<options>] [<revision-range>] [[--] <path>...]
 
 DESCRIPTION
 -----------

builtin/fast-export.c

@@ -33,7 +33,7 @@
 #include "gpg-interface.h"
 
 static const char *const fast_export_usage[] = {
-	N_("git fast-export [<rev-list-opts>]"),
+	N_("git fast-export [<options>] [<revision-range>] [[--] <path>...]"),
 	NULL
 };
 

t/t0450/adoc-help-mismatches

@@ -12,7 +12,6 @@ column
 credential
 credential-cache
 credential-store
-fast-export
 fast-import
 fetch-pack
 fmt-merge-msg