revert_to_git2_0.13.21_to_avoid_issues

This change is related to cargo-generate#461 and cargo-generate#455.
Both issues seem to be caused by the use of the newer `libgit2-sys` version.

An issue has been created for `git2` team: rust-lang/git2-rs#746

Author: taurr

Cargo.lock

@@ -15,7 +15,9 @@ console = "0.14.1"
 dialoguer = "0.8.0"
 dirs = "3.0.2"
 indicatif = "0.16.2"
-git2 = "0.13.22"
+# git2:0.13.22 pulls libgit2-sys:0.12.23+1.2.0, which causes errors on some systems
+git2 = "0.13.21"
+libgit2-sys = "=0.12.22+1.1.0"
 tempfile = "3.2.0"
 regex = "1.5.4"
 heck = "0.3.3"