git fatal error due to usage of shell escape character (^) on Windows

The ^ character escapes the following character in shell commands on Windows.

Therefore, grn's current commands fail with:

# drush -v rn 6.x-3.0 6.x-3.1
Executing: "git" show -s --pretty=format:%H "6.x-3.0"^{commit}
6.x-3.0 is not a valid Git tag.

# "git" show -s --pretty=format:%H "6.x-3.0"^{commit}
fatal: ambiguous argument '6.x-3.0{commit}': unknown revision or path not in the working tree.

The escaping character needs to be moved inside of the escaped shell argument string (%s), so it isn't interpreted as escaping character any longer.

Attached patch makes grn work on Windows.