From: Srujith A Poojary <poojarysrujith@gmail.com>

The commit IDs shown in a submodule diff header belong to the
submodule's object database. However, strbuf_add_unique_abbrev()
uses the_repository, so it checks the superproject's object
database when abbreviating those IDs.

Use the submodule repository when it is available, and fall back
to the repository from the diff options when it is not.

As the expected abbreviation depends on the hash algorithm used,
the test uses the test_oid mechanism to switch the expectation
between SHA-1 and SHA-256 repositories.

Signed-off-by: Srujith A Poojary <poojarysrujith@gmail.com>

Submitted-As: https://lore.kernel.org/git/pull.2288.v6.git.1793821554097.gitgitgadget@gmail.com
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