userdiff: add support for R programming language

Add userdiff patterns to support R programming language.

Also, add three userdiff tests for R programming language
files. These files define simple function and nested function,
with and without indentation.

Signed-off-by: Rodrigo Carvalho <rodrigorsdc@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4018/r-indent

@@ -0,0 +1,6 @@
+RIGHT <- function(a, b) {
+    c = mean(a, b)
+    d = c + 2
+    ChangeMe()
+    return (d)
+}

t/t4018/r-indent-nested

@@ -0,0 +1,10 @@
+LEFT = function(a, b) {
+    c = mean(a, b)
+    RIGHT = function(d, e) {
+        f = var(d, e)
+        g = f + 1
+        ChangeMe()
+        return (g)
+    }
+    return (RIGHT(2, 3))
+}

t/t4018/r-noindent

@@ -0,0 +1,6 @@
+RIGHT <- function(a, b) {
+c = mean(a, b)
+d = c + 2
+ChangeMe()
+return (c)
+}

userdiff.c

@@ -327,6 +327,10 @@ PATTERNS("python",
 	 "|[-+0-9.e]+[jJlL]?|0[xX]?[0-9a-fA-F]+[lL]?"
 	 "|[-+*/<>%&^|=!]=|//=?|<<=?|>>=?|\\*\\*=?"),
 	 /* -- */
+PATTERNS("r",
+	"^[ \t]*([a-zA-z][a-zA-Z0-9_.]*[ \t]*(<-|=)[ \t]*function.*)$",
+	/* -- */
+	"[^ \t]+"),
 PATTERNS("ruby",
 	 "^[ \t]*((class|module|def)[ \t].*)$",
 	 /* -- */