userdiff.c: add builtin fortran regex patterns

userdiff.c: add builtin fortran regex patterns

This adds fortran xfuncname and wordRegex patterns to the list of builtin
patterns.  The intention is for the patterns to be appropriate for all
versions of fortran including 77, 90, 95.  The patterns can be enabled by
adding the diff=fortran attribute to the .gitattributes file for the
desired file glob.

This also adds a new macro named IPATTERN which is just like the PATTERNS
macro except it sets the REG_ICASE flag so that case will be ignored.

The test code in t4018 and the docs were updated as appropriate.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>