Add basic Mercurial support to the watchman repo

Summary: Add an hgignore and arc support.

Test Plan: Check `arc which` in both hg and git.

Reviewers: wez

Reviewed By: wez

CC: fugalh

Differential Revision: https://phabricator.fb.com/D863836

--HG--
extra : amend_source : e2d5d22aaa50bd8ab18338007061b71f9cc1543b

.arcconfig

@@ -5,5 +5,6 @@
   "load"        : ["arcanist"],
   "copyright_holder" : "Facebook, Inc.",
   "git.default-relative-commit" : "origin/master",
+  "base" : "git:merge-base(origin/master), hg:gca(master)",
   "conduit_uri" : "https://phabricator.fb.com/api/"
 }

.gitignore

@@ -1,3 +1,4 @@
+# If you make a change to this file, make sure to also make it to .hgignore.
 Makefile.in
 aclocal.m4
 autom4te.cache/

.hgignore

@@ -0,0 +1,33 @@
+# If you make a change to this file, make sure to also make it to .gitignore.
+syntax: glob
+
+Makefile.in
+aclocal.m4
+autom4te.cache/
+configure
+depcomp
+install-sh
+missing
+config.log
+.deps/
+*.in
+Makefile
+*.o
+config.status
+watchman
+arcanist/.phutil_module_cache
+*.swp
+compile
+stamp-h1
+config.h*
+thirdparty/jansson/jansson_config.h
+compile
+tests/*.t
+.dirstamp
+*.a
+tests/integration/.watchman.*
+*.rpm
+config.sub
+config.guess
+configure.lineno
+python/bser/build