Trac #22504: Document git trac try

This is taken mostly verbatim from git-trac-command's README.md at
https://github.com/sagemath/git-trac-command

URL: https://trac.sagemath.org/22504
Reported by: paulmasson
Ticket author(s): Paul Masson
Reviewer(s): Daniel Krenn

src/doc/en/developer/git_trac.rst

@@ -357,12 +357,13 @@ This creates a new "merge" commit, joining your current branch and
     * Or you definitely need a feature that has been developed as part
       of another branch.
 
-A special case of merging is merging in the ``master`` branch. This
+A special case of merging is merging in the ``develop`` branch. This
 brings your local branch up to date with the newest Sage version. The
 above warning against unnecessary merges still applies, though. Try to
 do all of your development with the Sage version that you originally
-started with. The only reason for merging in the master branch is if
-you need a new feature or if your branch conflicts.
+started with. The only reason for merging in the ``develop`` branch is if
+you need a new feature or if your branch conflicts. See
+:ref:`section-git-update-latest` for details.
 
 
 .. _section-git_trac-collaborate:
@@ -532,8 +533,7 @@ end: git downloads both Alice's conflicting commit and her resolution.
 Reviewing
 =========
 
-This section gives an example how to review using the ``sage`` command. For an
-explanation of what should be checked by the reviewer, see
+For an explanation of what should be checked by the reviewer, see
 :ref:`chapter-review`.
 
 If you go to the `web interface to the Sage trac development server
@@ -552,3 +552,22 @@ to use the web interface:
   shows you what is being added, analogous to clicking on the
   "Branch:" field.
 
+To review tickets with minimal recompiling, start by building the "develop"
+branch, that is, the latest beta. Just checking out an older ticket would most
+likely reset the Sage tree to an older version, so you would have to compile
+older versions of packages to make it work. Instead, you can create an anonymous
+("detached HEAD") merge of the ticket and the develop branch using ::
+
+    $ git trac try 12345
+
+This will only touch files that are really modified by the ticket. In particular,
+if only Python files are changed by the ticket (which is true for most tickets)
+then you just have to run ``sage -b`` to rebuild the Sage library. If files other
+than Python have been changed, you must run ``make``. When you are finished
+reviewing, just check out a named branch, for example ::
+
+    $ git checkout develop
+
+If you want to edit the ticket branch (that is, add additional commits) you cannot
+use ``git trac try``. You must :ref:`section-git_trac-checkout` to get the actual ticket
+branch as a starting point.