From: Feng Wu <wufengwufengwufeng@gmail.com>

The loose object map stores entries keyed by the repository's storage
hash and the compatible hash.  ObjectMap::insert() accepts its two object
IDs in either order, but it currently checks only whether oid1 uses the
compatible hash algorithm.  If it does not, oid2 is assumed to be the
compatible ID without validating oid2's algorithm.

That means callers can pass two IDs with the same algorithm, or an ID
using an unknown algorithm, and have one of them silently treated as the
storage ID.  This does not match the map invariant that each entry must
contain exactly one storage hash and one compatible hash.

Make the invariant explicit by decoding both object ID algorithms and
rejecting unknown or mismatched pairs before inserting anything.  Introduce
ObjectMapInsertError with InvalidHashAlgorithm and MismatchedAlgorithms
variants for clear error reporting.

Update the existing tests to unwrap successful insertions, and add tests
for same-algorithm and unknown-algorithm inputs.

Signed-off-by: Feng Wu <wufengwufengwufeng@gmail.com>

Submitted-As: https://lore.kernel.org/git/pull.2350.git.git.1782489506255.gitgitgadget@gmail.com