color: Replace anonymous sentinel with named class

Replace the bare `object()` sentinel used for `_CHECK_CONSOLE` with an
instance of a dedicated `_CheckConsoleSentinel` class. This gives the
sentinel a meaningful repr and type, making it easier to identify in
debugging output and type checks compared to an opaque `<object>`.

Change-Id: I916521d47ba13207e29a2412e00f3576aff8afff
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/605021
Commit-Queue: Brian Gan <brgan@google.com>
Tested-by: Brian Gan <brgan@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

color.py

diff --git a/color.py b/color.py
index 4068634..53fadf0 100644
--- a/color.py
+++ b/color.py
@@ -84,9 +84,14 @@
 
 DEFAULT = None
 
+
+class _CheckConsoleSentinel:
+    """Sentinel for checking console coloring."""
+
+
 # Placholder value that indicates we need to check if the user is in an
 # interactive terminal session to determine if we turn on color or not.
-_CHECK_CONSOLE = object()
+_CHECK_CONSOLE = _CheckConsoleSentinel()
 
 # https://git-scm.com/docs/git-config#Documentation/git-config.txt-colorui
 _CONFIG_TO_COLOR_SETTING = {