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

README.md

@@ -274,9 +274,11 @@ are now tracked only in the [issues](https://git.eeqj.de/sneak/mfer/issues).
       tree later. Each file is downloaded to a temp file beside it, such as
       `.a.txt.tmp` for `a.txt`, then moved into place. A manifest is refused
       before any file is downloaded if it lists a file where fetch writes
-      another: at the temp file of a listed file, or at `index.mf` or
-      `.index.mf.tmp` at the top of the tree. Names are compared in any letter
-      case.
+      another: at another listed file or a directory one is in, at the temp file
+      of a listed file, or at `index.mf` or `.index.mf.tmp` at the top of the
+      tree. Names are compared in any letter case, on every filesystem, since on
+      a case-insensitive one `A.txt` and `a.txt` are one file; a directory two
+      listed files are in must be spelled alike in both.
 - `mfer fetch --require-signature <fingerprint> https://example.com/stuff/`
     - as above, but first refuses a manifest not signed by the key with that
       fingerprint, as `mfer check --require-signature` does, before downloading