getdeps optionally can get hg info from env var (#157)

[dgrnbrg-meta]

Summary: 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

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D34732506