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_test.go

@@ -1237,17 +1237,14 @@ func TestFetchRefusesListedTempName(t *testing.T) {
 }
 
 // TestFetchRefusesNamesEqualIgnoringCase fetches manifests that list two
-// 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.
+// 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.
 func TestFetchRefusesNamesEqualIgnoringCase(t *testing.T) {
 	t.Parallel()