Refactor Git/LFS request specs into shared examples

What does this MR do?

Part of #207869 (closed), extracted from !45892 (merged).

This allows us to reuse these specs in EE.

• The shared examples for Repositories::GitHttpController were moved without any changes.

• The LFS request specs were cleaned up and simplified a bit.

  • See the first commit for easier review, the second commit just corrects the indentation.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #207869 (closed)