commit: Fix `GetTreeEntries()` failing hard when reading a blob

The GetTreeEntries() will, given a commit and a path inside that commit, return all tree entries below that path. While behaviour is obvious in the case where the path does point to a subtree of the commit in question, it's less obvious in case it's pointing to a blob given that there cannot be any tree entries returned by such a query. While it would be feasible to just return an error in that case given that the query is misformed, we historically handled such queries gracefully by returning no results at all.

This behaviour broke when converting the tree entries helper to use the new catfile interfaces: the old Batch interface used to return a NotFoundError if the expected type didn't match the actual type, and we explicitly handle that case to gracefully fail and swallow the error. Wtih the conversion, We do not check for the type of the object returned by the catfile process anymore, but just blindly try to parse it as a tree object even if it refers to a blob. Luckily, this regression was catched by Rails' test suite.

Fix the regression by properly checking for the object type again: in case it is not a tree, we just return nothing at all. While at it, this commit also makes sure to always discard objects in case we fail with an error such that the object reader is not dirty on failure.

Changelog: fixed

Fixes: 31205084 (commit: Convert tree entry helpers to accept catfile readers, 2021-10-12)

Part of create-stage#12938 (closed)

Fixes #3861 (closed)