Summary:
`MountingTransaction` encapsulates all artifacts of `ShadowTree` commit, particularly list of mutations and meta-data.
We will rely on this heavily in the coming diffs.
Reviewed By: JoshuaGross
Differential Revision: D15021795
fbshipit-source-id: 811da7afd7b929a34a81aa66566193d46bbc34f8
Summary: Because it's kinda more logical and we will rely on this in comming diffs.
Reviewed By: mdvacca
Differential Revision: D14587124
fbshipit-source-id: 94ae9410b4ffeffd0fcb4da4a0518f0bb0d2ba63