Fix RuboCop lints in StagedFile

Add some parentheses around the parameters for two method definitions in
`StagedFile`.

Change-Id: If78f7ee668256c24a02c9333558b7e6fd2081d99
Reviewed-on: https://gerrit.causes.com/25157
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>