restic: ignore filenames which are not IDs when expanding a prefix

Some backends generate additional files for each existing file, e.g.

1234567890abcdef1234567890abcdef1234567890abcdef1234567890abcdef
1234567890abcdef1234567890abcdef1234567890abcdef1234567890abcdef.sha256

For some commands this leads to an "multiple IDs with prefix" error when
trying to reference a snapshot.

changelog/unreleased/pull-3943

@@ -0,0 +1,9 @@
+Enhancement: Ignore additional files in repository
+
+Some commands like `find` and `restore` could become confused by additional
+files in the repository. This could cause restic to fail with an `multiple IDs
+with prefix "12345678" found` error. These commands now ignore additional
+files.
+
+https://github.com/restic/restic/pull/3943
+https://forum.restic.net/t/which-protocol-should-i-choose-for-remote-linux-backups/5446/17

internal/restic/backend_find.go

@@ -3,6 +3,8 @@ package restic
 import (
 	"context"
 	"fmt"
+
+	"github.com/restic/restic/internal/debug"
 )
 
 // A MultipleIDMatchesError is returned by Find() when multiple IDs with a
@@ -31,6 +33,13 @@ func Find(ctx context.Context, be Lister, t FileType, prefix string) (string, er
 	defer cancel()
 
 	err := be.List(ctx, t, func(fi FileInfo) error {
+		// ignore filename which are not an id
+		_, err := ParseID(fi.Name)
+		if err != nil {
+			debug.Log("unable to parse %v as an ID", fi.Name)
+			return nil
+		}
+
 		if len(fi.Name) >= len(prefix) && prefix == fi.Name[:len(prefix)] {
 			if match == "" {
 				match = fi.Name