changelog: add class to represent parsed changelog revisions Currently, changelog entries are parsed into their respective components at read time. Many operations are only interested in a subset of fields of a changelog entry. The parsing and storing of all the fields adds avoidable overhead. This patch introduces the "changelogrevision" class. It takes changelog raw text and exposes the parsed results as attributes. The code for parsing changelog entries has been moved into its construction function. changelog.read() has been modified to use the new class internally while maintaining its existing API. Future patches will make revision parsing lazy. We implement the construction function of the new class with __new__ instead of __init__ so we can use a named tuple to represent the empty revision. This saves overhead and complexity of coercing later versions of this class to represent an empty instance. While we are here, we add a method on changelog to obtain an instance of the new type. The overhead of constructing the new class regresses performance of revsets accessing this data: author(mpm) 0.896565 0.929984 desc(bug) 0.887169 0.935642 105% date(2015) 0.878797 0.908094 extra(rebase_source) 0.865446 0.922624 106% author(mpm) or author(greg) 1.801832 1.902112 105% author(mpm) or desc(bug) 1.812438 1.860977 date(2015) or branch(default) 0.968276 1.005824 author(mpm) or desc(bug) or date(2015) or extra(rebase_source) 3.656193 3.743381 Once lazy parsing is implemented, these revsets will all be faster than before. There is no performance change on revsets that do not access this data. There /could/ be a performance regression on operations that perform several changelog reads. However, I can't think of anything outside of revsets and `hg log` (basically the same as a revset) that would be impacted.