ostree: provide command description in a better place

This is similar idea as
https://github.com/projectatomic/rpm-ostree/pull/916/commits/5c0bf889155573feec108501ce849ac09a0c9567,

The duplicated description is now removed, and the description
of the command is now displayed beneath the Usage.

For example:
ostree cat -h will output the following:

"Usage:
  ostree cat [OPTION?] COMMIT PATH...

Concatenate contents of files"

Closes: #1267
Approved by: cgwalters

src/ostree/ot-builtin-cat.c

@@ -57,7 +57,7 @@ cat_one_file (GFile         *f,
 gboolean
 ostree_builtin_cat (int argc, char **argv, OstreeCommandInvocation *invocation, GCancellable *cancellable, GError **error)
 {
-  g_autoptr(GOptionContext) context = g_option_context_new ("COMMIT PATH... - Concatenate contents of files");
+  g_autoptr(GOptionContext) context = g_option_context_new ("COMMIT PATH...");
   g_autoptr(OstreeRepo) repo = NULL;
   if (!ostree_option_context_parse (context, options, &argc, &argv, invocation, &repo, cancellable, error))
     return FALSE;