Use the detailed commit graph automatically in terminals that draw it

The detailed commit graph is only drawn for users who set
gui.commitGraphStyle to 'detailed'. Many users of terminals that can
draw it will never find out about it. A terminal can't be asked whether
it draws a given character, but it does tell us its name and version
when tcell asks for them with XTVERSION at startup.

Add an 'auto' value and make it the default. It uses the detailed graph
in kitty from 0.36.2 and in Ghostty from 1.0.0 on. These are the first
versions that draw all of the symbols. Everywhere else it stays with
the classic graph. This includes tmux, because tmux answers XTVERSION
itself. So far, WezTerm draws the symbols only in its nightly builds, so
it isn't detected until a release has them. VS Code isn't detected
either, because it only draws the symbols with GPU acceleration.

The expected output of the integration tests has the classic graph, so
pin it in their config. Otherwise they would fail when run with a
visible UI in one of these terminals.

Log the terminal's name and version at startup, to make it possible to
find out why 'auto' picked what it did.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

test/default_test_config/config.yml

@@ -12,6 +12,9 @@ gui:
     - black
   # Not important in tests but it creates clutter in demos
   showRandomTip: false
+  # The expected output of the tests draws the commit graph with box drawing
+  # characters, whichever terminal a test runs in
+  commitGraphStyle: classic
   animateExplosion: false # takes too long
 git:
   # We don't want to run any periodic background git commands because it'll introduce race conditions and flakiness.