diff: --color-moved: rename "dimmed_zebra" to "dimmed-zebra"

The --color-moved "dimmed_zebra" mode (with an underscore) is an
anachronism. Most options and modes are hyphenated. It is more difficult
to type and somewhat more difficult to read than those which are
hyphenated. Therefore, rename it to "dimmed-zebra", and nominally
deprecate "dimmed_zebra".

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -274,10 +274,11 @@ zebra::
 	painted using either the 'color.diff.{old,new}Moved' color or
 	'color.diff.{old,new}MovedAlternative'. The change between
 	the two colors indicates that a new block was detected.
-dimmed_zebra::
+dimmed-zebra::
 	Similar to 'zebra', but additional dimming of uninteresting parts
 	of moved code is performed. The bordering lines of two adjacent
 	blocks are considered interesting, the rest is uninteresting.
+	`dimmed_zebra` is a deprecated synonym.
 --
 
 --word-diff[=<mode>]::

diff.c

@@ -275,10 +275,12 @@ static int parse_color_moved(const char *arg)
 		return COLOR_MOVED_ZEBRA;
 	else if (!strcmp(arg, "default"))
 		return COLOR_MOVED_DEFAULT;
+	else if (!strcmp(arg, "dimmed-zebra"))
+		return COLOR_MOVED_ZEBRA_DIM;
 	else if (!strcmp(arg, "dimmed_zebra"))
 		return COLOR_MOVED_ZEBRA_DIM;
 	else
-		return error(_("color moved setting must be one of 'no', 'default', 'zebra', 'dimmed_zebra', 'plain'"));
+		return error(_("color moved setting must be one of 'no', 'default', 'zebra', 'dimmed-zebra', 'plain'"));
 }
 
 int git_diff_ui_config(const char *var, const char *value, void *cb)

t/t4015-diff-whitespace.sh

@@ -1223,7 +1223,7 @@ test_expect_success 'plain moved code, inside file' '
 	test_cmp expected actual
 '
 
-test_expect_success 'detect permutations inside moved code -- dimmed_zebra' '
+test_expect_success 'detect permutations inside moved code -- dimmed-zebra' '
 	git reset --hard &&
 	cat <<-\EOF >lines.txt &&
 		long line 1
@@ -1271,7 +1271,7 @@ test_expect_success 'detect permutations inside moved code -- dimmed_zebra' '
 	test_config color.diff.newMovedDimmed "normal cyan" &&
 	test_config color.diff.oldMovedAlternativeDimmed "normal blue" &&
 	test_config color.diff.newMovedAlternativeDimmed "normal yellow" &&
-	git diff HEAD --no-renames --color-moved=dimmed_zebra --color |
+	git diff HEAD --no-renames --color-moved=dimmed-zebra --color |
 		grep -v "index" |
 		test_decode_color >actual &&
 	cat <<-\EOF >expected &&