Never report index.mf at the top of the base as unlisted in check (closes #229)

gen and fetch write the manifest as index.mf at the top of the tree.
check left out only the manifest file it was given, so given a URL, or a
manifest stored elsewhere, it reported that index.mf as not in the
manifest, and --no-extra-files failed a tree fetch had just verified.
check now drops that one report for index.mf at the top of the base
directory. A manifest that lists index.mf still has it checked, and
index.mf in a subdirectory or fetch's leftover .index.mf.tmp are still
reported. The change is in package cli, which owns the default manifest
name; the Checker in package mfer is unchanged.

Model: opus-5-5

README.md

@@ -304,6 +304,9 @@ are now tracked only in the [issues](https://git.eeqj.de/sneak/mfer/issues).
       `--no-extra-files` each one is a failure instead. A named pipe, socket or
       device, a symlink to a directory and a broken symlink are never reported,
       since gen cannot list them
+    - never reports `index.mf` at the top of the base directory as a file the
+      manifest does not list, even when given another manifest or a URL, since
+      gen and fetch write the manifest there
 - `mfer freshen` / `mfer freshen .`
     - rewrites `index.mf` to list the files now under the directory holding it,
       or under the one `--base` names, hashing only the files that are new or