resource/repository_file: ignore commit-related fields during acctest import step

[anGie44]

Addresses test failures since commit info is no longer set in Read per #466:

--- FAIL: TestAccGithubRepositoryFile_basic (15.20s)
    testing.go:654: Step 2 error: ImportStateVerify attributes not equivalent. Difference is shown below. Top is actual, bottom is expected.
        
        (map[string]string) {
        }
        
        
        (map[string]string) (len=3) {
         (string) (len=13) "commit_author": (string) (len=9) "Test User",
         (string) (len=12) "commit_email": (string) (len=60) "[email protected]",
         (string) (len=14) "commit_message": (string) (len=34) "Update tf-acc-test-file-7r6ww6pfrt"
        }

--- FAIL: TestAccGithubRepositoryFile_branch (12.28s)
    testing.go:654: Step 2 error: ImportStateVerify attributes not equivalent. Difference is shown below. Top is actual, bottom is expected.
        
        (map[string]string) {
        }
        
        
        (map[string]string) (len=3) {
         (string) (len=13) "commit_author": (string) (len=9) "Test User",
         (string) (len=12) "commit_email": (string) (len=60) "[email protected]",
         (string) (len=14) "commit_message": (string) (len=34) "Update tf-acc-test-file-u2iquispnf"
        }

--- FAIL: TestAccGithubRepositoryFile_committer (15.33s)
    testing.go:654: Step 2 error: ImportStateVerify attributes not equivalent. Difference is shown below. Top is actual, bottom is expected.
        
        (map[string]string) {
        }
        
        
        (map[string]string) (len=3) {
         (string) (len=13) "commit_author": (string) (len=14) "Terraform User",
         (string) (len=12) "commit_email": (string) (len=21) "[email protected]",
         (string) (len=14) "commit_message": (string) (len=20) "Managed by Terraform"
        }