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

On a case-insensitive filesystem such paths are one name. Of two files,
one replaced the other and fetch exited 0. For a file and a directory
another file is in, fetch stopped partway with a non-zero exit, leaving
a partial tree. For two spellings of one directory, both files landed
in one directory, one under a spelling the manifest does not list, and
check reported that file as not in the manifest.

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

Model: opus-5-5

internal/cli/fetch_test.go

@@ -1237,14 +1237,17 @@ func TestFetchRefusesListedTempName(t *testing.T) {
 }
 
 // TestFetchRefusesNamesEqualIgnoringCase fetches manifests that list two
-// paths that are one name on a case-insensitive filesystem: two files,
-// at the top of the tree and in a directory, a file and a directory
-// another file is in, and two directories. Fetching such a manifest
-// there would replace one with the other, so fetch must refuse it on
-// every filesystem before it creates the destination or requests any
-// file. A file and a directory with the same name, and a file listed
-// twice, are refused the same way. A manifest whose names differ in more
-// than letter case is fetched in full.
+// paths that are one name on a case-insensitive filesystem. Fetched
+// there, of two such files, at the top of the tree or in a directory,
+// one replaces the other and fetch exits 0. A file and a directory
+// another file is in stop fetch partway with a non-zero exit, leaving a
+// partial tree. Two spellings of one directory put both files in one
+// directory, one of them under a spelling the manifest does not list,
+// and check reports that file as not in the manifest. So fetch must
+// refuse each on every filesystem before it creates the destination or
+// requests any file. A file and a directory with the same name, and a
+// file listed twice, are refused the same way. A manifest whose names
+// differ in more than letter case is fetched in full.
 func TestFetchRefusesNamesEqualIgnoringCase(t *testing.T) {
 	t.Parallel()