add config for spawning watchman with sh in certain macOS environments
add config for spawning watchman with sh in certain macOS environments

Summary:
We decided to teach watchman to spawn itself with sh because on CI we need
permissions to access the filesystem, and manually clicking buttons on
all those hosts is not feisable.

We could sign watchman and explicitly grant watchman permissions to access the
filesystem, but this will require some changes to our release process and a
manual review. So spawning with sh which has the permissions we need on
CI was just easier.

it turns out sh does not have the permissions to open files in certain
directories with more restrictive ACLs like Documents, Desktop etc.

This caused internal users with repos in these directories to no longer be
able to use watchman. Additionally, folks in open source worlds were broken.

For now to be able to unblock watchman use in CI we add a config which we
can enable there to launch watchman with sh. in all other environments
we will launch watchman the old way.

Reviewed By: chadaustin

Differential Revision: D37125444

fbshipit-source-id: 13cdf0fd688dc7a9c5344c861f34669f0d7c5208

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