update: fix latest_tag in homebrew/brew update

[shcai97]

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brew update fetches commit history since latest_tag for Homebrew/brew. However, git ls-remote sorts the tag in lexicographical order by default. So the current command does not always get the latest tag.
For the moment,git ls-remote --tags --refs -q origin | tail -n1 | cut -f2 outputs refs/tags/2.4.9 instead of refs/tags/2.4.16. This commit provides a fix.

[shcai97]

I thought this piece of code is for updating brew in all situations.
After I read again, I found it only applies for those use a non-official source. (HOMEBREW_BREW_GIT_REMOTE set)
But I think this change still apply.

[maxim-belkin]

Thanks for the PR, @shcai97.
Indeed, git ls-remote --tags --refs -q origin returns refs in the lexicographical order. The --sort flag was added in Git 2.18, and the oldest version supported by Homebrew is 2.14.3 (see HOMEBREW_MINIMUM_GIT_VERSION in brew.sh file).
Homebrew had a similar situation in the past when it moved from using git tag ... --sort to git tag ... | sort ..., see d4311fd. So, until the minimum supported Git version is newer than or equal to 2.18, we have to use | sort approach. Can you update your PR to do that?

[shcai97]

Hi, Maxim. I think it should be good now.
One problem is that the fixed line only outputs the version number, so I add one more line to add refs/tags/ in front of the number to keep the same format.
I am not familiar with the --shallow-since. I have tested it on my machine, give version number only (2.4.16 instead of refs/tags/2.4.16) also works. I am not sure whether the additional line is necessary.

[MikeMcQuaid]

Thanks so much for your excellent first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @shcai97!