team: mark slug as computed when name changes

[bendrucker]

This configures Terraform to mark github_team.*.slug as computed any time name changes, ensuring that it shows any resource that is dependent on slug as requiring an update.

I spotted this need when using terraform import with an existing team that had a different name than what was declared in configuration. Other resources in the same configuration (team_sync_group_mapping in my case) that depended on that slug still showed the current slug in the plan. Without this hint from CustomizeDiff, Terraform can't know that the computed value of slug will be different after the name is updated and the value in state (from the import) should not be used.

I used customdiff.Sequence as a general pattern here so that additional CustomizeDiffFuncs can be added for other fields. However, it could easily be removed too.

A test that has another resource that depends on slug provides coverage here. Using another team and sticking a slug in the description was convenient. Without the customized diff, the test fails:

==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run TestAccGithubTeamUpdateName -timeout 120m
?   	github.com/terraform-providers/terraform-provider-github	[no test files]
=== RUN   TestAccGithubTeamUpdateName
=== RUN   TestAccGithubTeamUpdateName/marks_the_slug_as_computed_when_the_name_changes
=== RUN   TestAccGithubTeamUpdateName/marks_the_slug_as_computed_when_the_name_changes/with_an_anonymous_account
    resource_github_team_test.go:199: anonymous account not supported for this operation
=== RUN   TestAccGithubTeamUpdateName/marks_the_slug_as_computed_when_the_name_changes/with_an_individual_account
    resource_github_team_test.go:203: individual account not supported for this operation
=== RUN   TestAccGithubTeamUpdateName/marks_the_slug_as_computed_when_the_name_changes/with_an_organization_account
    testing.go:654: Step 1 error: errors during apply:

        Error: Provider produced inconsistent final plan

        When expanding the plan for github_team.other to include new values learned so
        far during apply, provider "github" produced an invalid new value for
        .description: was cty.StringVal("tf-acc-kpzaa"), but now
        cty.StringVal("tf-acc-updated-kpzaa").

        This is a bug in the provider, which should be reported in the provider's own
        issue tracker.

--- FAIL: TestAccGithubTeamUpdateName (8.71s)
    --- FAIL: TestAccGithubTeamUpdateName/marks_the_slug_as_computed_when_the_name_changes (8.71s)
        --- SKIP: TestAccGithubTeamUpdateName/marks_the_slug_as_computed_when_the_name_changes/with_an_anonymous_account (0.00s)
        --- SKIP: TestAccGithubTeamUpdateName/marks_the_slug_as_computed_when_the_name_changes/with_an_individual_account (0.00s)
        --- FAIL: TestAccGithubTeamUpdateName/marks_the_slug_as_computed_when_the_name_changes/with_an_organization_account (8.71s)
FAIL
FAIL	github.com/terraform-providers/terraform-provider-github/github	8.970s
FAIL
make: *** [testacc] Error 1

With the custom diff the test passes.

[jcudit]

This is a neat helper. Thanks for bringing it into the project!

[jcudit]

Much appreciation for getting this edge case covered. I feel blind to this use case and am thankful to have contributions like this 🎉