Merge branch 'fix-multiple-ref-prefix' into 'master'

Extract the longest-matching ref from a commit path when multiple matches occur

### What does this MR do?

This MR extracts the longest-matching ref from a commit path. In cases when there are multiple refs that prefix the path, the ref name is ambiguous. Using the heuristic that the longest-matching ref seems like a sensible default.

### Why was this MR needed?

Suppose there is a branch named `release/app` and a tag named `release/app/v1.0.0`. Suppose `README.md` exists in the root directory.

Let's suppose the path passed in is `release/app/v1.0.0/README.md`. There are two possible ways to interpret the ref and path:

1. ref = `release/app`, path = `v1.0.0/README.md`
2. ref = `release/app/v1.0.0`, path = `README.md`

The crux of the issue is that there is ambiguity which one is correct; both could be real possibilities. In the current implementation of `extract_ref`, GitLab gets confused and tries neither: it uses ref = `release` and path = `app/v1.0.0/README.md`. Since the file does not exist, it returns 404.

### What are the relevant issue numbers?

Closes #1839

See merge request !859