config: fix segfault when parsing "core.abbrev" without repo

The "core.abbrev" config allows the user to specify the minimum length when abbreviating object hashes. Next to the values "auto" and "no", this config also accepts a concrete length that needs to be bigger or equal to the minimum length and smaller or equal to the hash algorithm's hex length. While the former condition is trivial, the latter depends on the object format used by the current repository. It is thus a variable upper boundary that may either be 40 (SHA-1) or 64 (SHA-256).

This has two major downsides. First, the user that specifies this config must be aware of the object hashes that its repository use. If they want to configure the value globally, then they cannot pick any value in the range [41, 64] if they have any repository that uses SHA-1. If they did, Git would error out when parsing the config.

Second, and more importantly, parsing "core.abbrev" crashes when outside of a Git repository because we dereference the_hash_algo to figure out its hex length. Starting with c8aed5e8 (repository: stop setting SHA1 as the default object hash, 2024-05-07) though, we stopped initializing the_hash_algo outside of Git repositories.

Fix both of these issues by not making it an error anymore when the given length exceeds the hash length. Instead, if we have a repository, then we truncate the length to the maximum length of the_hash_algo. Otherwise, we simply leave the abbreviated length intact and store it as-is. This is equivalent to the logic in parse_opt_abbrev_cb() and is handled just fine by repo_find_unique_abbrev_r(). In practice, we should never even end up using default_abbrev without a repository anyway given that abbreviating object IDs to unique prefixes requires us to have access to an object database.

Reported-by: Kyle Lippincott spectral@google.com Signed-off-by: Patrick Steinhardt ps@pks.im