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

internal/cli/entry_test.go

@@ -1358,6 +1358,33 @@ func TestCheckReportsUnreadableIndexMFAtTopOfBase(t *testing.T) {
 	assert.Contains(t, testStderr(t, opts), "ERROR: "+defaultManifestName)
 }
 
+// TestCheckReportsUpperCaseIndexMFAtTopOfBase checks a tree holding
+// INDEX.MF at its top against a manifest that does not list it. Only the
+// name index.mf, in exactly that letter case, is left out, so check reports
+// INDEX.MF. The tree is in memory, where the two names are two files
+// whatever the letter case rules of the host's filesystem.
+func TestCheckReportsUpperCaseIndexMFAtTopOfBase(t *testing.T) {
+	t.Parallel()
+
+	upperCase := strings.ToUpper(defaultManifestName)
+
+	fs := afero.NewMemMapFs()
+	writeTestFile(t, fs, testFile1, "hello")
+
+	opts := testOpts([]string{
+		testApp, cmdGenerate, "-q", "-o", testManifest, testDir,
+	}, fs)
+	require.Equal(t, 0, runCLI(opts), "stderr: %s", testStderr(t, opts))
+
+	writeTestFile(t, fs, filepath.Join(testDir, upperCase), "not listed")
+
+	opts = testOpts([]string{
+		testApp, cmdCheck, testFlagNoExtra, testFlagBase, testDir, testManifest,
+	}, fs)
+	assert.Equal(t, 1, runCLI(opts), "stderr: %s", testStderr(t, opts))
+	assert.Contains(t, testStderr(t, opts), "EXTRA: "+upperCase)
+}
+
 func TestGenerateAtomicWriteNoTempFileOnSuccess(t *testing.T) {
 	t.Parallel()