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From:   Junio C Hamano <gitster@pobox.com>
To:     Alyssa Ross <hi@alyssa.is>
Cc:     git@vger.kernel.org, Denton Liu <liu.denton@gmail.com>
Subject: Re: [PATCH] diff: fix --merge-base with annotated tags
In-Reply-To: <20231001151845.3621551-1-hi@alyssa.is> (Alyssa Ross's message of
        "Sun, 1 Oct 2023 15:18:45 +0000")
References: <20231001151845.3621551-1-hi@alyssa.is>
Date:   Mon, 02 Oct 2023 11:54:52 -0700
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Alyssa Ross <hi@alyssa.is> writes:

> Checking early for OBJ_COMMIT excludes other objects that can be
> resolved to commits, like annotated tags.  If we remove it, annotated
> tags will be resolved and handled just fine by
> lookup_commit_reference(), and if we are given something that can't be
> resolved to a commit, we'll still get a useful error message, e.g.:
>
>> error: object 21ab162211ac3ef13c37603ca88b27e9c7e0d40b is a tree, not a commit
>> fatal: no merge base found

Interesting.  0f5a1d44 (builtin/diff-index: learn --merge-base,
2020-09-20) claims that it took inspiration from "git diff A...B"
but forgot that it needs to accept any commit-ish.

With a devil's advocate hat on, I have to wonder if it is really a
useful error message to spew a long hexadecimal string when the user
would certainly have gave a more mnemonic HEAD^{tree} or something,
but the original message does not say which command line argument it
did not like anyway, so the patch is a net improvement.

Will queue.  Thanks.