fetch refuses a manifest whose paths differ only in letter case (closes #154)

On a case-insensitive filesystem, such as the macOS default, A.txt and
a.txt are one file: fetch downloaded one, replaced it with the other and
exited 0, leaving a tree check rejects. A file and a directory whose
names differ only in case did the same.

The existing name-clash check now also records each listed file, and
each directory one is in, among the names fetch writes. A listed file at
a name already taken, or a directory spelled differently from one
already there, refuses the manifest on every filesystem, before the
destination is created or any file requested. The message names both
paths.

Model: opus-5-5

internal/cli/fetch.go

@@ -466,17 +466,18 @@ func fetchManifest(
 }
 
 // checkNoNameClash returns an error if files lists a file, or a directory
-// a file is in, under a name where fetch writes another file: the temp
-// file it downloads a listed file to, or, at the top of the tree, the
-// saved manifest or its temp file. fetch would remove or replace what is
+// a file is in, under a name where fetch writes another file: another
+// listed file, a directory another listed file is in, the temp file it
+// downloads a listed file to, or, at the top of the tree, the saved
+// manifest or its temp file. fetch would remove or replace what is
 // listed there, or fail partway, leaving a tree check rejects. Names are
-// compared ignoring case, since on a case-insensitive filesystem INDEX.MF
-// and index.mf are one file.
+// compared ignoring case, on every filesystem, since on a
+// case-insensitive one A.txt and a.txt are one file.
 func checkNoNameClash(files []*mfer.MFFilePath) error {
 	sep := string(filepath.Separator)
 
-	// written maps each name fetch writes, other than the listed files
-	// themselves, in lower case, to the file it writes there.
+	// written maps each name fetch writes, in lower case, to the file or
+	// directory it writes there.
 	written := map[string]string{
 		defaultManifestName:              "the saved manifest",
 		tempPathFor(defaultManifestName): "the saved manifest's temp file",
@@ -488,14 +489,26 @@ func checkNoNameClash(files []*mfer.MFFilePath) error {
 	}
 
 	for _, f := range files {
-		// Look up each directory on the file's path, then the file itself.
-		parts := strings.Split(strings.ToLower(filepath.Clean(f.GetPath())), sep)
+		// Look up and add each directory on the file's path, then the
+		// file itself. Only the same directory, spelled alike, may
+		// already be there.
+		parts := strings.Split(filepath.Clean(f.GetPath()), sep)
+		last := len(parts) - 1
 
 		for i := range parts {
-			what, ok := written[strings.Join(parts[:i+1], sep)]
-			if ok {
-				return fmt.Errorf("%w: %s (%s)", errNameClash, f.GetPath(), what)
+			name := strings.Join(parts[:i+1], sep)
+
+			what := "the directory " + name
+			if i == last {
+				what = "the file " + f.GetPath()
 			}
+
+			other, ok := written[strings.ToLower(name)]
+			if ok && (i == last || other != what) {
+				return fmt.Errorf("%w: %s (%s)", errNameClash, f.GetPath(), other)
+			}
+
+			written[strings.ToLower(name)] = what
 		}
 	}