Update data source for repository file: prevent tf panic on non-existent file and make the branch default consistent with GitHub API

[galargh]

Resolves #1132
Resolves #1131

Description

This PR updates the data source for repository files.

It prevents terraform from panicking when trying to retrieve info about a non-existent file. With this change, in such a case, an empty data source will get returned - similarly to how ref data source behaves.

It also changes the default value for branch argument from main to the name of the default branch of the requested repository. The new behaviour is how GitHub API itself behaves.

I also added a new attribute ref which, after apply, holds the name of the branch the file is associated with. In practice, if a branch was explicitly provided, then ref = branch. Otherwise, ref = the name of the default branch of the requested repository.

Finally, it simplified the commit info retrieval. It was possible because I noticed that commit_sha is never set on the data source - I suspect it was copied over from the resource and just left in place.

I updated the markdown entry which describes how to configure the resource.

Testing

• TF_ACC=1 go test -v ./... -run ^TestAccGithubRepositoryFileDataSource

[douernesto]

This would solve a lot of issues!

[kfcampbell]

@galargh do you have any interest in resolving this merge conflicts? I can do a pass at it if you'd prefer as well.

[galargh]

@galargh do you have any interest in resolving this merge conflicts? I can do a pass at it if you'd prefer as well.

Sure, no problem. All conflicts should be resolved now.

[kfcampbell]

Hmm, tests for TestDataSourceGithubRepositoryFileRead are failing for me. Here is some example test output:

    data_source_github_repository_file_test.go:373: 
        	Error Trace:	/home/kfcampbell/github/dev/terraform-provider-github/github/data_source_github_repository_file_test.go:373
        	Error:      	Not equal: 
        	            	expected: "here-goes-content-of-our-glorious-config.json"
        	            	actual  : ""
        	            	
        	            	Diff:
        	            	--- Expected
        	            	+++ Actual
        	            	@@ -1 +1 @@
        	            	-here-goes-content-of-our-glorious-config.json
        	            	+
        	Test:       	TestDataSourceGithubRepositoryFileRead/using_user_as_owner_if_just_name_is_passed
    data_source_github_repository_file_test.go:374: 
        	Error Trace:	/home/kfcampbell/github/dev/terraform-provider-github/github/data_source_github_repository_file_test.go:374
        	Error:      	Not equal: 
        	            	expected: "test-repo/test-file.json"
        	            	actual  : ""
        	            	
        	            	Diff:
        	            	--- Expected
        	            	+++ Actual
        	            	@@ -1 +1 @@
        	            	-test-repo/test-file.json
        	            	+
        	Test:       	TestDataSourceGithubRepositoryFileRead/using_user_as_owner_if_just_name_is_passed

Can you reproduce this?

[galargh]

I've just realised I was only running TestAccGithubRepositoryFileDataSource before 🤦

I now fixed [TestDataSourceGithubRepositoryFileRead](https://github.com/integrations/terraform-provider-github/pull/1129/commits/1a8161ff9d5e3529e5b1c6cafac8a370adb59d98) too. I needed to update GitHub client mock responses which totally makes sense since the ds is making slightly different requests now.

I also ran go fmt on modified files.

[kfcampbell]

@galargh thank you so much for contributing and following up! I'll get this merged and released shortly.