minimum commit id length when fixing a patch

[Igor Pylypiv]

If your patch fixes a bug in a specific commit, e.g. you found an
issue using git bisect,
please use the ‘Fixes:’ tag with the first 12 characters of the SHA-1
ID, and the one line summary.
For example:

Fixes: e21d2170f366 ("video: remove unnecessary platform_set_drvdata()")

Quote from: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#describe-your-changes

Example from github:
https://github.com/torvalds/linux/commit/5f348fa35a28b19cf7ee1eaa420757341a724c86

On Tue, Jun 6, 2017 at 1:34 PM, Kevin Wilson <wkevils at gmail.com> wrote:
> Hi,
> I want to send a patch to LKML fixing something wrong which was caused
> by other patch.
>
> Form what I understand, it is a good practice to mention the commit
> title and the commit ID.
>
> A commit ID is very long. My question is: when mentioning in my commit
> log the patch that caused the problem, what is the minimum commit id
> length which I should use when presenting the patch that caused the
> problem ? is such commit length is documented anywhere ?
>
> Regards,
> Kevin
>
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