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

[DanielEScherzer]

Crosslink libgit2/libgit2#2910
There is an init option https://docs.rs/git2/latest/git2/struct.RepositoryInitOptions.html#method.external_template to use an external template, and https://docs.rs/git2/latest/git2/struct.RepositoryInitOptions.html#method.template_path to add a custom path, but the actual reading from the environment

There is GIT_REPOSITORY_OPEN_FROM_ENV as a flag but no flag for init