Fixed for libgit2 1.8

[How2PlayGithub]

Fixed for libgit2:1.8.0-2

The error functions and enumeration values have been renamed for consistency. The giterr_ functions and values prefix have been renamed to be prefixed with git_error_; similarly, the GITERR_ constants have been renamed to be prefixed with GIT_ERROR_. The old enumerations and macros will be retained for backward compatibility for the foreseeable future.

https://github.com/libgit2/libgit2/blob/main/docs/changelog.md

[How2PlayGithub]

I had this error today and found that I just needed to change it to giterr_clear for it make

[ken-matsui]

Thank you for fixing this! It seems that libgit2 is not following SemVer... Just a nit, please keep the newline before the namespace end. Otherwise, it looks good to me.

[ken-matsui]

Thank you!