Replace calls to git.ExecSafe with specific use cases

Created by: ryanslade

Instead of calling directly into git.ExecSafe, we should instead call into functions that show the intent.

For example: https://github.com/sourcegraph/sourcegraph/blob/main/cmd/frontend/graphqlbackend/externallink/repository.go#L47

We should instead be calling a function named something like git.GetDefaultBranchShort.

Once all calls are changed, remove git.ExecSafe