Update libgit2 to 0.28
treec6d744c97f90b84049e9bfda1e9c2233080de1b2
-rw-r--r-- 1327 .editorconfig
-rw-r--r-- 9 .gitattributes
-rw-r--r-- 600 .gitignore
-rw-r--r-- 1056 .gitmodules
-rw-r--r-- 700 .mailmap
-rw-r--r-- 354 .tgitconfig
drwxr-xr-x - Languages
-rw-r--r-- 3975 README.md
-rw-r--r-- 2323 appveyor.yml
-rw-r--r-- 6138 architecture.txt
-rw-r--r-- 3626 build.txt
drwxr-xr-x - contrib
drwxr-xr-x - doc
drwxr-xr-x - ext
-rw-r--r-- 1261 release.txt
drwxr-xr-x - src
drwxr-xr-x - test