Generate revertibles for commits incoming from a merge

[noencke]

Description

This corrects some behavior in the (currently alpha) branching APIs. Currently, merging one or more commits into a target branch from a source branch does not generate revertibles on the target branch. This PR updates it so that each merge commit fires a "commitApplied" event and generates the proper revertible.

[msfluid-bot]

⯅ @fluid-example/bundle-size-tests: +345 Bytes

Metric Name	Baseline Size	Compare Size	Size Diff
aqueduct.js	461.42 KB	461.45 KB	⯅ +35 Bytes
azureClient.js	559.03 KB	559.08 KB	⯅ +49 Bytes
connectionState.js	680 Bytes	680 Bytes	■ No change
containerRuntime.js	261.68 KB	261.69 KB	⯅ +14 Bytes
fluidFramework.js	404.79 KB	404.85 KB	⯅ +64 Bytes
loader.js	134.17 KB	134.19 KB	⯅ +14 Bytes
map.js	42.43 KB	42.44 KB	⯅ +7 Bytes
matrix.js	146.8 KB	146.8 KB	⯅ +7 Bytes
odspClient.js	526.18 KB	526.23 KB	⯅ +49 Bytes
odspDriver.js	97.8 KB	97.82 KB	⯅ +21 Bytes
odspPrefetchSnapshot.js	42.76 KB	42.78 KB	⯅ +14 Bytes
sharedString.js	163.79 KB	163.8 KB	⯅ +7 Bytes
sharedTree.js	395.25 KB	395.31 KB	⯅ +57 Bytes
Total Size	3.31 MB	3.31 MB	⯅ +345 Bytes

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Baseline commit: 5575b1e

Generated by 🚫 dangerJS against 576d41f

[Josmithr]

Does this include a revertible for the merge commit itself, or just the commits that made up the merged branch?

[noencke]

Good question. There is no such thing as a merge commit, at least not as a primitive. Merge simply applies all the divergent changes of one branch to another branch (after rebasing them). The git-style "squash merge commit" is accomplished by the user running the merge inside of a transaction (because the transaction will squash all the commits that came in from the other branch). I expect that's going to be a pretty common pattern. Unfortunately we don't support it yet (you'll crash if you try) because there is some rebaser work that needs to be done first (I think it's in progress though!).

[Josmithr]

Primarily reviewed the docs. Someone with a better understanding of the underlying logic should review too :)