getdeps optionally can get hg info from env var (#911)
getdeps optionally can get hg info from env var (#911)

Summary:
X-link: https://github.com/rsocket/rsocket-cpp/pull/911

X-link: https://github.com/facebookexperimental/rust-shed/pull/32

X-link: https://github.com/facebookincubator/reindeer/pull/3

X-link: https://github.com/fairinternal/AIRStore/pull/36

X-link: https://github.com/facebookincubator/velox/pull/1197

Pull Request resolved: https://github.com/facebookincubator/mvfst/pull/242

X-link: https://github.com/facebookexperimental/eden/pull/116

X-link: https://github.com/facebookincubator/fizz/pull/75

X-link: https://github.com/facebookincubator/katran/pull/157

X-link: https://github.com/facebook/watchman/pull/1011

X-link: https://github.com/facebook/wangle/pull/205

X-link: https://github.com/facebook/proxygen/pull/401

X-link: https://github.com/facebook/openr/pull/129

X-link: https://github.com/facebook/fbzmq/pull/35

X-link: https://github.com/facebook/fb303/pull/26

X-link: https://github.com/facebook/bistro/pull/59

X-link: https://github.com/facebook/folly/pull/1734

X-link: https://github.com/facebook/fboss/pull/113

Adds an environment variable to getdeps to provide `hg` info to avoid calling `hg` directly.

When using `getdeps` inside a containerized environment (which we need to build Research Super Cluster tooling with the correct linker attributes), `getdeps` fails because of unregistered mercurial extensions in the `hgrc`.

This allows `getdeps` to be useable in an environment where the mercurial extensions used in a project aren't installed/available.

Reviewed By: vivekspai

Differential Revision: D34732506

fbshipit-source-id: 6475088fbf3323347095ca5af8a902382cf8f9d0

facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst