Update Tree-hygiene.md (#167699)

Closes https://github.com/flutter/flutter/issues/167688.

Feel free to suggest other minor changes, or we can make more changes
after, either/or.

docs/contributing/Tree-hygiene.md

@@ -11,6 +11,12 @@ Flutter other than the actual writing of the code. For guidance on
 designing APIs, documenting, and formatting your code, see the
 [Style guide for Flutter repo](Style-guide-for-Flutter-repo.md) document.
 
+Across the document we use the word "tree" to mean "the health state of flutter/flutter repository". Tree status is shown:
+
+- On our [build dashboard](https://flutter-dashboard.appspot.com/)
+- On every PR (referred to as "Tree Status")
+- In the [tree-status](https://discord.com/channels/608014603317936148/613398423093116959) Discord channel
+
 ## Overview
 
 The general process for submitting code to a Flutter repository is as follows:
@@ -301,12 +307,6 @@ the repository yet, then wait for one of the project maintainers to submit it fo
 
 If you do have access, add the "autosubmit" label. A bot will land the PR for you.
 
-
-### Squashing commits
-
-When you squash commits, by default, GitHub will concatenate all your commit messages to form a unified commit message.  This often yields an overly verbose commit message with many unhelpful entries (e.g. "fix typo").  Please double-check (and hand-edit if necessary) your commit message before merging such that the message contains a helpful description of the overall change.
-
-
 ## Tree breakage (regressions in functionality)
 
 If a check-in has caused a regression on the main branch (sometimes "master") for any of the flutter repositories,