bin/local-pull: Clarify docs, add more tests for corrupted local pulls

I was reading the pull-local command docs and realized it was somewhat unclear
that `--untrusted` *only* applied to local repo pulls; in other words that we
always treat non-local pulls as untrusted.

Tweak the docstring, and add tests that verify this explicitly.

Closes: #1130
Approved by: jlebon

src/ostree/ot-builtin-pull-local.c

@@ -47,7 +47,7 @@ static int opt_depth = 0;
 static GOptionEntry options[] = {
   { "remote", 0, 0, G_OPTION_ARG_STRING, &opt_remote, "Add REMOTE to refspec", "REMOTE" },
   { "disable-fsync", 0, 0, G_OPTION_ARG_NONE, &opt_disable_fsync, "Do not invoke fsync()", NULL },
-  { "untrusted", 0, 0, G_OPTION_ARG_NONE, &opt_untrusted, "Do not trust source", NULL },
+  { "untrusted", 0, 0, G_OPTION_ARG_NONE, &opt_untrusted, "Do not verify checksums of local sources (always enabled for HTTP pulls)", NULL },
   { "bareuseronly-files", 0, 0, G_OPTION_ARG_NONE, &opt_bareuseronly_files, "Reject regular files with mode outside of 0775 (world writable, suid, etc.)", NULL },
   { "require-static-deltas", 0, 0, G_OPTION_ARG_NONE, &opt_require_static_deltas, "Require static deltas", NULL },
   { "gpg-verify", 0, 0, G_OPTION_ARG_NONE, &opt_gpg_verify, "GPG verify commits (must specify --remote)", NULL },

src/ostree/ot-builtin-pull.c

@@ -57,7 +57,7 @@ static GOptionEntry options[] = {
    { "require-static-deltas", 0, 0, G_OPTION_ARG_NONE, &opt_require_static_deltas, "Require static deltas", NULL },
    { "mirror", 0, 0, G_OPTION_ARG_NONE, &opt_mirror, "Write refs suitable for a mirror and fetches all refs if none provided", NULL },
    { "subpath", 0, 0, G_OPTION_ARG_FILENAME_ARRAY, &opt_subpaths, "Only pull the provided subpath(s)", NULL },
-   { "untrusted", 0, 0, G_OPTION_ARG_NONE, &opt_untrusted, "Do not trust (local) sources", NULL },
+   { "untrusted", 0, 0, G_OPTION_ARG_NONE, &opt_untrusted, "Do not verify checksums of local sources (always enabled for HTTP pulls)", NULL },
    { "bareuseronly-files", 0, 0, G_OPTION_ARG_NONE, &opt_bareuseronly_files, "Reject regular files with mode outside of 0775 (world writable, suid, etc.)", NULL },
    { "dry-run", 0, 0, G_OPTION_ARG_NONE, &opt_dry_run, "Only print information on what will be downloaded (requires static deltas)", NULL },
    { "depth", 0, 0, G_OPTION_ARG_INT, &opt_depth, "Traverse DEPTH parents (-1=infinite) (default: 0)", "DEPTH" },