userdiff: add built-in pattern for rust

This adds xfuncname and word_regex patterns for Rust, a quite
popular programming language. It also includes test cases for the
xfuncname regex (t4018) and updated documentation.

The word_regex pattern finds identifiers, integers, floats and
operators, according to the Rust Reference Book.

Cc: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4018-diff-funcname.sh

@@ -43,6 +43,7 @@ diffpatterns="
 	php
 	python
 	ruby
+	rust
 	tex
 	custom1
 	custom2

t/t4018/rust-fn

@@ -0,0 +1,5 @@
+pub(self) fn RIGHT<T>(x: &[T]) where T: Debug {
+    let _ = x;
+    // a comment
+    let a = ChangeMe;
+}

t/t4018/rust-impl

@@ -0,0 +1,5 @@
+impl<'a, T: AsRef<[u8]>>  std::RIGHT for Git<'a> {
+
+    pub fn ChangeMe(&self) -> () {
+    }
+}

t/t4018/rust-struct

@@ -0,0 +1,5 @@
+#[derive(Debug)]
+pub(super) struct RIGHT<'a> {
+    name: &'a str,
+    age: ChangeMe,
+}

t/t4018/rust-trait

@@ -0,0 +1,5 @@
+unsafe trait RIGHT<T> {
+    fn len(&self) -> u32;
+    fn ChangeMe(&self, n: u32) -> T;
+    fn iter<F>(&self, f: F) where F: Fn(T);
+}