color: Treat "true" and "yes" as "auto", not "always"

Per the git documentation for color.ui [1], setting color.ui to "true"
(or "yes") should behave identically to "auto", enabling color only
when output is written to a terminal or an active pager. Previously,
repo was equating "true" and "yes" with "always", which caused color
escape codes to be emitted unconditionally, even when output was piped
or redirected.

Replace the duplicated string-matching logic in SetDefaultColoring and
Coloring.__init__ with a single CONFIG_TO_COLOR_SETTING dict that maps
all git color config values to their behavior. This makes the mapping
easy to verify against the git docs and impossible to get out of sync
between the two call sites.

Added tests for SetDefaultColoring and Coloring.__init__ covering
all color mode values (auto, true, yes, always, never, no, false),
case insensitivity, TTY vs pipe behavior, active pager detection,
and unrecognised input.

[1] https://git-scm.com/docs/git-config#Documentation/git-config.txt-colorui

Bug: 295841573
Change-Id: I8a04b9c7e4154de37ed7518c010233039e0afdc9
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/602981
Tested-by: Brian Gan <brgan@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Brian Gan <brgan@google.com>

tests/test_color.py

@@ -14,6 +14,8 @@
 
 """Unittests for the color.py module."""
 
+from unittest import mock
+
 import pytest
 import utils_for_test
 
@@ -24,9 +26,14 @@ import git_config
 @pytest.fixture
 def coloring() -> color.Coloring:
     """Create a Coloring object for testing."""
+    return _make_coloring("always")
+
+
+def _make_coloring(default_state: str) -> color.Coloring:
+    """Set the default color mode and return a Coloring using test config."""
     config_fixture = utils_for_test.FIXTURES_DIR / "test.gitconfig"
     config = git_config.GitConfig(config_fixture)
-    color.SetDefaultColoring("true")
+    color.SetDefaultColoring(default_state)
     return color.Coloring(config, "status")
 
 
@@ -72,3 +79,76 @@ def test_Color_Parse_empty_entry(coloring: color.Coloring) -> None:
     assert val == "\033[2;34;47m"
     val = coloring._parse("empty", "green", "white", "bold")
     assert val == "\033[1;32;47m"
+
+
+class TestSetDefaultColoring:
+    """Tests for SetDefaultColoring."""
+
+    def test_none_leaves_default_unchanged(self) -> None:
+        color.DEFAULT = "auto"
+        color.SetDefaultColoring(None)
+        assert color.DEFAULT == "auto"
+
+    @pytest.mark.parametrize(
+        "value, expected",
+        (
+            # auto/true/yes all store their lowercase form.
+            ("auto", "auto"),
+            ("Auto", "auto"),
+            ("true", "true"),
+            ("True", "true"),
+            ("yes", "yes"),
+            ("Yes", "yes"),
+            # "always" stores as "always".
+            ("always", "always"),
+            ("Always", "always"),
+            # never/no/false store their lowercase form.
+            ("never", "never"),
+            ("no", "no"),
+            ("false", "false"),
+        ),
+    )
+    def test_maps_to_expected(self, value: str, expected: str) -> None:
+        color.SetDefaultColoring(value)
+        assert color.DEFAULT == expected
+
+    def test_unrecognised_leaves_default_unchanged(self) -> None:
+        color.DEFAULT = "auto"
+        color.SetDefaultColoring("garbage")
+        assert color.DEFAULT == "auto"
+
+
+class TestColoringInit:
+    """Tests for Coloring.__init__ color mode logic."""
+
+    @pytest.mark.parametrize(
+        "state, isatty, pager_active, expected",
+        (
+            # "always" enables color unconditionally.
+            ("always", False, False, True),
+            # "never" disables color unconditionally.
+            ("never", True, True, False),
+            # auto/true/yes enable color only on a TTY or active pager.
+            ("auto", True, False, True),
+            ("auto", False, False, False),
+            ("auto", False, True, True),
+            ("true", True, False, True),
+            ("true", False, False, False),
+            ("true", False, True, True),
+            ("yes", True, False, True),
+            ("yes", False, False, False),
+            ("yes", False, True, True),
+        ),
+    )
+    def test_color_mode(
+        self,
+        state: str,
+        isatty: bool,
+        pager_active: bool,
+        expected: bool,
+    ) -> None:
+        with mock.patch("os.isatty", return_value=isatty), mock.patch(
+            "pager.active", pager_active
+        ):
+            c = _make_coloring(state)
+        assert c.is_on is expected