Fixing problems with project overview

What does this MR do and why?

This fixes a few bugs on the project overview when the redirect_with_ref_type feature flag is enabled.

Fix problems around commit messages and last commits:

• There is a graphql query with ref_type with the incorrect case (should be uppercase), this didn't seem to be related to the bugs here but we're fixing it anyways.
• Make ref_type case insensitive for api requests (allowing frontend to always uppercase ref_type)
• make project overview implicitly request a branch ref_type

Fix problems around buttons for adding files:

• make head_tree always refer to a branch (the default branch) rather than situationally to a tag

Screenshots or screen recordings

Pay particular attention to:

• the last commit widget is showing the wrong commit in the before image
• the commit messages do not render for the files that are not in the tag in the before image
• there is a README button on the before image even though there isn't a README in the default branch

Before	After

How to set up and validate locally

0. Enable the redirect_with_ref_type feature flag (e.g Feature.enable(:redirect_with_ref_type))
1. Create a repository with a README
2. Create a branch main2 that does not have a readme For example

git checkout -b main2 &&
git rm README.md &&
git commit -m "removed readme"

3. Add another file to main2

echo "newfile" > newfile &&
git add newfile &&
git commit -m "Add newfile" &&
git push origin main2

4. Create a tag main2 that does have README For example

git checkout main &&
git tag main2 &&
git push --tags

5. Make main2 the default branch of the repository (Settings -> Repository -> Branch defaults)
6. Check the project overview to see that
   • the latest commit widget shows the message "removed readme"
   • the commit message next newfile loads
   • There is an Add README button

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #419973 (closed)