Set commit id when ref used explicitly (#26447)

Set commit id when ref used explicitly (#26447)

In the `RepoRefForAPI()` context function `CommitID` is not set if `ref`
is used. It is set correctly for other if/else branches where `Commit`
is set. It doesn't appear that any routes that use `RepoRefForAPI()`
also use `CommitID` but that may be the case in the future.

## Changes
- Sets `ctx.Repo.CommitID` when `ref` is explicitly used for api routes
that use `RepoRefForAPI()`