hg: Display formatted user name.

[Niloth-p]

Mercurial: Format the user name to hide the user's email in the summary line of the message.
Motivation: Updating example screenshots in integration docs.

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Mercurial's ctx.user() returns the username.

The username can be set from these sources:

• env var $HGUSER
• [ui] username in .hgrc.
• System username as a fallback

The username could be in any of the following formats, and may have extra spaces anywhere:

• username <email> (the recommended format)
• username
• <email>

I've added a parse_user function which supports all the formats, and strips extra spaces.
It could have been written in a couple of lines, but I've kept the function elaborate and used intermediate variables for readability.

How did you test this PR?
I did not.

Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

[timabbott]

Doesn't Python have an emailaddress module for parsing these correctly? There's no reason to write custom regexes to parse standard formats.