gittest: Convert `NewRepositoryName()` to always create ".git" suffix

The NewRepositoryName() function computes a random repository hash for the caller. This function has two different modes: one to create a bare repository path with ".git" suffix, and one non-bare one without it.

There is basically not a single instance though where we'd want this function to create a repository path without the ".git" suffix. In production systems we always have that suffix as we only ever have bare repositories. Which means that tests which do use a non-bare repository path don't match our typical hashed storage layout. This is something we should avoid though: it's preferable to have our test data as close to production data as possible.

This MR converts all remaining callers that create non-bare repository paths to stop doing that so that we can drop the parameter in the final commit.