Stabilize SHA-256 support

[weihanglo]

In git2@0.21.0 we have added SHA-256 support behind an unstable Cargo feature unstable-sha256.

git2-rs/Cargo.toml

Lines 37 to 52 in dffaf27

	# Experimental SHA256 OID support,
	# reflecting upstream libgit2's GIT_EXPERIMENTAL_SHA256.
	#
	# This is an ABI-breaking change.
	# Future releases with this feature may introduce breakages without notice
	# Use at your own risk.
	#
	# Library authors:
	# DO NOT enable this feature by default in your dependencies.
	# Due to Cargo's additive features,
	# downstream users cannot deactivate it once enabled.
	#
	# NOTE: libgit2's builtin SHA256 implementation is not thread-safe.
	# Enable the `https` feature to route hashing through thread-safe OpenSSL impl.
	# See https://github.com/rust-lang/git2-rs/issues/1255.
	unstable-sha256 = ["libgit2-sys/unstable-sha256"]

It is done this way because the upstream libgit2 v1.9 also marks it as experimental. The plan in upstream is that there will be a stable SHA-256 support (+ reftable I believe) in libgit2 v2, and it is coming: libgit2/libgit2#7149 (reply in thread). We should move it out from unstable when upstream is ready.

cc #1090

[weihanglo]

When stabilizing it, we might also consider to move away from *_ext method to always include an ObjectFormat to avoid misuse. Still in discussion and should weight on the amount of churns though.