Support or allow support for GIT_TEMPLATE_DIR

[dwijnand]

Git has an environment variable called GIT_TEMPLATE_DIR that it uses to customise the behaviour of git init. It would appear that neither libgit2 nor git2-rs support this. See the downstream issues in cargo for more info, history/motivation and reproductions:

• cargo-new ignores $GIT_TEMPLATE_DIR cargo#5420
• Cargo build with dependencies fails on Cygwin referring to git-core/templates cargo#1295