Handle sparse git repo without erroring.

[chachi]

Workaround for libgit2 spuriously erroring causing cargo to refuse to build.

Fixes #8746 and #7876

Upstream fix: libgit2/libgit2#5656

[rust-highfive]

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[ehuss]

I'm not sure, but I don't think this is needed with #8778?

Also, I'm not sure if #7876 is actually fixed. The original report didn't include a working reproduction, so it is difficult to guess what the original reporter was running into.

[chachi]

Indeed. Looks like it works without this anymore. Excited that got fixed so quickly! Closing.