Revert "upgrade of libgit2 0.27.1 (#27241)"

[staticfloat]

This reverts commit 4a303b3.

This broke the buildbots as it introduced a dependency upon libssl.so again, see discussions:

• upgrade of libgit2 0.27.1 #27241 (comment)
• 4a303b3#commitcomment-29274618

Since this broke nightlies, let's get this merged quick, then we can iterate on figuring out the solution on a feature branch. The basic test for a working fix is to build it locally, then check to see if libgit2.so has a dependency on libssl.so.

[vtjnash]

introduced a dependency upon libssl.so again

Note that afaik, the issue isn't the dependence on libssl – we expect this and build a copy of it for this purpose. However, cmake can't find the right version without a patch (https://github.com/JuliaLang/julia/pull/27241/files#diff-979517b9a4ab3894b541d602a9b2d346)

[staticfloat]

I think you’re talking about libssh2, not libssl

[vtjnash]

Ah, right. I was confused since it also broke the (other) buildbots due to libssh2 😜