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

Summary:
X-link: rsocket/rsocket-cpp#911

X-link: facebookexperimental/rust-shed#32

X-link: facebookincubator/reindeer#3

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

X-link: facebookincubator/velox#1197

X-link: facebook/mvfst#242

X-link: facebook/sapling#116

X-link: facebookincubator/fizz#75

X-link: facebookincubator/katran#157

X-link: facebook/watchman#1011

X-link: facebook/wangle#205

X-link: facebook/proxygen#401

X-link: facebook/openr#129

Pull Request resolved: #35

X-link: facebook/fb303#26

X-link: facebookarchive/bistro#59

X-link: facebook/folly#1734

X-link: facebook/fboss#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

Author: dgrnbrg-meta

build/fbcode_builder/getdeps/fetcher.py

@@ -525,12 +525,15 @@ def get_fbsource_repo_data(build_options) -> FbsourceRepoData:
     if cached_data:
         return cached_data
 
-    cmd = ["hg", "log", "-r.", "-T{node}\n{date|hgdate}"]
-    env = Env()
-    env.set("HGPLAIN", "1")
-    log_data = subprocess.check_output(
-        cmd, cwd=build_options.fbsource_dir, env=dict(env.items())
-    ).decode("ascii")
+    if "GETDEPS_HG_REPO_DATA" in os.environ:
+        log_data = os.environ["GETDEPS_HG_REPO_DATA"]
+    else:
+        cmd = ["hg", "log", "-r.", "-T{node}\n{date|hgdate}"]
+        env = Env()
+        env.set("HGPLAIN", "1")
+        log_data = subprocess.check_output(
+            cmd, cwd=build_options.fbsource_dir, env=dict(env.items())
+        ).decode("ascii")
 
     (hash, datestr) = log_data.split("\n")