bisync: fix listings missing concurrent modifications - fixes #8359

Before this change, there was a bug affecting listing files when:

- a given bisync run had changes in the 2to1 direction
AND
- the run had NO changes in the 1to2 direction
AND
- at least one of the changed files changed AGAIN during the run
(specifically, after the initial march and before the transfers.)

In this situation, the listings on one side would still retain the prior version
of the changed file, potentially causing conflicts or errors.

This change fixes the issue by making sure that if we're updating the listings
on one side, we must also update the other. (We previously tried to skip it for
efficiency, but this failed to account for the possibility that a changed file
could change again during the run.)

cmd/bisync/bisync_test.go

@@ -746,6 +746,16 @@ func (b *bisyncTest) runTestStep(ctx context.Context, line string) (err error) {
 	case "test-func":
 		b.TestFn = testFunc
 		return
+	case "concurrent-func":
+		b.TestFn = func() {
+			src := filepath.Join(b.dataDir, "file7.txt")
+			dst := "file1.txt"
+			err := b.copyFile(ctx, src, b.replaceHex(b.path2), dst)
+			if err != nil {
+				fs.Errorf(src, "error copying file: %v", err)
+			}
+		}
+		return
 	case "fix-names":
 		// in case the local os converted any filenames
 		ci.NoUnicodeNormalization = true