Reject secret mv onto the same secret under another name (closes #78)

On a case-insensitive filesystem (the macOS default) "Foo" and "foo" name
one secret, so `secret mv --force Foo foo` removed the destination, which
was the source, and lost the secret with every version. Between vaults the
copy replaced the source, and removing the source then removed the copy.

Both kinds of move now compare the two secret directories with
os.SameFile before changing anything and reject the move if they are one,
with or without --force. The tests give one secret two names with
symbolic links on the real filesystem.

Model: opus-5-5

README.md

@@ -139,6 +139,9 @@ matching.
 
 Moves or renames a secret within the current vault.
 - Fails if the destination already exists
+- Fails if the destination is the source under another name, such as `foo`
+  for `Foo` on a case-insensitive filesystem (the macOS default); there, to
+  change only the case of a name, move the secret to a third name first
 - Preserves all versions and metadata
 
 ### Version Management