build: mark bisync golden test files as generated

Before this change, GitHub showed the bisync golden test files
(`cmd/bisync/testdata/*/golden/`) in full in pull request diffs, where a
new test scenario can add hundreds of lines of expected output.

After this change, they are marked `linguist-generated`, like `MANUAL.*`
and `rclone.1`, so GitHub collapses them in diffs and leaves them out of
the language statistics. They are still one click away in a diff.

.gitattributes

@@ -8,6 +8,7 @@ lib/http/templates/** text eol=lf
 # Ignore generated files in GitHub language statistics and diffs
 /MANUAL.* linguist-generated=true
 /rclone.1 linguist-generated=true
+cmd/bisync/testdata/*/golden/** linguist-generated=true
 
 # Don't fiddle with the line endings of test data
 **/testdata/** -text