Update libgit2-sys to 0.7.3

[heirecka]

It imports a newer version of libgit2 which includes fixes to build
against LibreSSL 2.7.x.

[rust-highfive]

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[kennytm]

Hi, thanks for the PR. This update has already been taken care of in #51917 which will be merged within days, so I'm closing this in favor of the other one.

[heirecka]

Sorry, I wasn't aware of that PR.