Bug 1395752 - Define and use sparse profile for Sphinx docs; r=dustin

The Sphinx documentation only needs access to a relatively small number
of files in the repo in order to be generated. It is a good candidate
for using sparse profiles.

This commit defines and uses a "sphinx-docs" sparse profile containing
only the files relevant to Sphinx documentation generation.

There are some quirks with the profile:

* All moz.build files are included. This bloats the profile
  by >1000 files. Worse, it realizes directories that have no business
  being realized. This clutters the checkout and makes it harder to
  find things. There is a moz.build reader that knows how to retrive
  file data from Mercurial. We could use that. This feels like follow-up
  fodder.

* All mach_commands.py files are included. `mach help` says you can do
  things that you aren't able to do in the sparse checkout. There isn't
  a good way to add all *.py files while excluding mach_commands.py
  files. We /could/ do it with regular expressions. But those are slow.
  Let's leave it as is for now and come up with a better solution later.

MozReview-Commit-ID: 7yiqGGE1nAh

--HG--
extra : rebase_source : c148040ea3618e8bfdd369b6f48fc60c6d179285
extra : source : b76e2f6204b20de137f2566dff8121ff3abe5760

build/sparse-profiles/sphinx-docs

@@ -0,0 +1,17 @@
+%include build/sparse-profiles/mach
+
+[include]
+# Code for generating docs.
+glob:tools/docs/**
+
+# Potential docs sources
+glob:**/*.rst
+
+# Python API docs.
+glob:**/*.py
+
+# moz.build files are read to discover location of docs.
+glob:**/moz.build
+
+# Read to set the version of the docs.
+path:config/milestone.txt

taskcluster/ci/source-test/doc.yml

@@ -18,6 +18,7 @@ doc-generate:
         command: >
             cd /builds/worker/checkouts/gecko &&
             ./mach doc --outdir docs-out --no-open --archive
+        sparse-profile: sphinx-docs
     when:
         files-changed:
             - '**/*.py'
@@ -40,6 +41,7 @@ doc-upload:
     run:
         using: run-task
         command: cd /builds/worker/checkouts/gecko && ./mach doc --upload --no-open
+        sparse-profile: sphinx-docs
     scopes:
         - secrets:get:project/releng/gecko/build/level-{level}/gecko-docs-upload
     when: