userdiff: expand detected chunk headers for css

The regex used for the CSS builtin diff driver in git is only
able to show chunk headers for lines that start with a number,
a letter or an underscore.

However, the regex fails to detect classes (starts with a .), ids
(starts with a #), :root and attribute-value based selectors (for
example [class*="col-"]), as well as @based block-level statements
like @page,@keyframes and @media since all of them, start with a
special character.

Allow the selectors and block level statements to begin with these
special characters.

Signed-off-by: Sohom Datta <sohom.datta@learner.manipal.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4018/css-attribute-value-selector

@@ -0,0 +1,4 @@
+[class*="RIGHT"] {
+    background : #000;
+    border : 10px ChangeMe #C6C6C6;
+}

t/t4018/css-block-level-@-statements

@@ -0,0 +1,10 @@
+@keyframes RIGHT {
+    from {
+        background : #000;
+        border : 10px ChangeMe #C6C6C6;
+    }
+    to {
+        background : #fff;
+        border : 10px solid #C6C6C6;
+    }
+}

t/t4018/css-class-selector

@@ -0,0 +1,4 @@
+.RIGHT {
+    background : #000;
+    border : 10px ChangeMe #C6C6C6;
+}

t/t4018/css-id-selector

@@ -0,0 +1,4 @@
+#RIGHT {
+    background : #000;
+    border : 10px ChangeMe #C6C6C6;
+}

t/t4018/css-root-selector

@@ -0,0 +1,4 @@
+:RIGHT {
+    background : #000;
+    border : 10px ChangeMe #C6C6C6;
+}

userdiff.c

@@ -200,7 +200,7 @@ PATTERNS("csharp",
 	 "|[-+*/<>%&^|=!]=|--|\\+\\+|<<=?|>>=?|&&|\\|\\||::|->"),
 IPATTERN("css",
 	 "![:;][[:space:]]*$\n"
-	 "^[_a-z0-9].*$",
+	 "^[:[@.#]?[_a-z0-9].*$",
 	 /* -- */
 	 /*
 	  * This regex comes from W3C CSS specs. Should theoretically also