Bump libgit2 dependency to latest upstream

[alerque]

Unblocks projects that link against native libgit2 for other reasons
from being able to upgrade.

[alerque]

Note for projects that need early access to this before it lands in a release, you can use this PR from your project by leaving your vergen and git2 dependencies as they are but adding this segment for Cargo.toml:

[patch.crates-io.vergen]
git = "https://github.com/alerque/vergen"
branch = "bump-libgit2"

I'll keep the branch active until a release that includes it lands and hence when your builds fail because the branch went away the next thing to do will be to bump vergen to make it go again.

[CraZySacX]

I'll cut a release today after this runs through the pipelines.

[codecov]

Codecov Report

Merging #145 (dd068cc) into master (4367de6) will increase coverage by 0.55%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #145      +/-   ##
==========================================
+ Coverage   87.76%   88.32%   +0.55%     
==========================================
  Files          10       10              
  Lines         474      471       -3     
==========================================
  Hits          416      416              
+ Misses         58       55       -3     

Impacted Files	Coverage Δ
src/config.rs	98.21% <0.00%> (+4.99%)	⬆️

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