changegroup: document deltaparent's choice of previous revision As part of debugging low-level changegroup generation, I came across what I initially thought was a weird behavior: changegroup v2 is choosing the previous revision in the changegroup as a delta base instead of p1. I was tempted to rewrite this to use p1, as p1 will delta better than prev in the common case. However, I realized that taking p1 as the base would potentially require resolving a revision fulltext and thus require more CPU for e.g. server-side processing of getbundle requests. This patch tweaks the code comment to note the choice of behavior. It also notes there is room for a flag or config option to tweak this behavior later: using p1 as the delta base would likely make changegroups smaller at the expense of more CPU, which could be beneficial for things like clone bundles.