Small Gitlab::Git fixes for Gitaly

What does this MR do?

• Add missing attr_accessor to Gitlab::Git::Conflict::File
• Use String#end_with? instead of String#ends_with?. The former is in Ruby's core lib, so is more flexible.

Are there points in the code the reviewer needs to double check?

No

Why was this MR needed?

Things I noticed I needed while working on gitaly!470 (merged)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

gitaly#788 (closed)