Keep the protocol/domain on one line and the path on another (in this case path is <80 chars)?

I've probably gone this route at least once. Perhaps more-ideal is to not put URLs (or, generally, giant blocks of explanatory text about context) into the raw source code, and leave a brief summary and a reference to a task instead. Then the details/URLs/etc could be updated or expanded upon without changing the code.

Slightly more broadly, we seem to have three remaining callsites in the code that use --debug:

'log --debug --limit 1 --template={parents} --rev %s',

This can be replaced with hg log --limit 1 --template '{p1node} {p2node}' --rev ... or similar since I added these keywords in ~2012, although they were deprecated in 2018; see https://patchwork.mercurial-scm.org/patch/36338/. The new syntax is {p1.node} and {p2.node} in recent versions of Mercurial.

'annotate --debug --changeset --rev %s -- %s',

This can be replaced with hg annotate --template '{lines % "{node}\n"}' --rev ... -- ... or similar. This callsite also doesn't call filterMercurialDebugOutput(), soooo...

'log --debug --template %s --limit %d --rev %s -- %C',
'{node};{parents}\\n',

This can be replaced with log --template '{node};{p1node} {p2node}\n' ...` or similar, per above.

With these three adjustments, filterMercurialDebugOutput() could be removed (I think?) and all of the associated garbage and tests could be deleted.

On navigating the version stuff, I think the pathway would be:

• Figure out the first version of Mercurial that has {p1node} and {p2node}. I think I upstreamed those patches in ~2012ish, but could be off.
• Make that the new minimum version of Mercurial for Phabricator. This happens in PhabricatorBinariesSetupCheck. The current minimum version is 1.9, with versions 2.1 and 2.2 forbidden because of buggy behavior.
• Figure out the first version of Mercurial that has {p1.node} and {p2.node}.
• Add a p1/p2 capability PhutilMercurialBinaryAnalyzer in Arcanist, then test for that capability to decide whether to use {p1node} or {p1.node}.

(DiffusionLowLevelMercurialPathsQuery has an example of using PhutilBinaryAnalyzer to do a Mercurial capability test.)

Ah thank you for those details for the better change. I was planning to play around with what would be needed to remove the --debug flag(s) but I hadn't yet gone into the details. I'll take a look at these changes. From quick testing on recent mercurial versions (5.8) it looks like the output does not include the local revision number, e.g instead of
693:8b39f63eb209dd2bdfd4bd3d0721a9e38d75a6d3 -1:0000000000000000000000000000000000000000
it prints out
8b39f63eb209dd2bdfd4bd3d0721a9e38d75a6d3 0000000000000000000000000000000000000000

I started a draft change in D21679. I also have changes for rARC but haven't yet created a diff for that (or yet figured out the exact versions to define)

According to their changelog the {p1node} style was added in 2.4 (2012) and was deprecated in 4.9 (2019) in favor of the new {p1.node} format
https://www.mercurial-scm.org/wiki/WhatsNew/Archive#Mercurial_2.4_.282012-11-01.29

However the use of --template for the hg annotate command does not appear to work until ~4.5 (2018). I say "appear to work" because although it does output what looks like expected results,

1. I couldn't find this behavior listed in their changelog, and
2. The use of --template/-T doesn't actually appear in the help output for annotate in version 4.5

It is both non-functional and non-documented in at least version 4.0 (2016) though I haven't bothered checking the major versions in between.