userdiff: add tests for Fortran xfuncname regex

The Fortran userdiff patterns, introduced in 909a5494f8 (userdiff.c: add
builtin fortran regex patterns, 2010-09-10), predate the test
infrastructure for xfuncname patterns, introduced in bfa7d01413 (t4018:
an infrastructure to test hunk headers, 2014-03-21).

Add tests for the Fortran xfuncname patterns. The test
't/t4018/fortran-comment-keyword' documents a shortcoming of the regex
that is fixed in a subsequent commit.

While at it, add descriptive comments for the different parts of the
regex.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4018/fortran-external-function

@@ -0,0 +1,9 @@
+function RIGHT(a, b) result(c)
+
+integer, intent(in) :: ChangeMe
+integer, intent(in) :: b
+integer, intent(out) :: c
+
+c = a+b
+
+end function RIGHT