Apply a 2-character indent to in-tree tooltool manifests to make
them easier to read, and to make the formatting more consistent
so automating updates is simpler.
Modern editors will maintain json indentation. The only long
lines we have are already over 80 characters, so the extra space
shouldn't create new long lines.
Also update mercurial installer script to generate json with
the same indentation, even though its output is temporary.
Tooltool itself was updated to generate manifests with this
indentation in Bug 1325225.
MozReview-Commit-ID: DKj6nL9OENv
192455dcb9