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()`

modules/context/api.go

@@ -340,6 +340,7 @@ func RepoRefForAPI(next http.Handler) http.Handler {
 				return
 			}
 			ctx.Repo.Commit = commit
+			ctx.Repo.CommitID = ctx.Repo.Commit.ID.String()
 			ctx.Repo.TreePath = ctx.Params("*")
 			next.ServeHTTP(w, req)
 			return