vchoi_fork

[svn-r2424] snapshot needs a version of diff that supports the -I (ignore pattern)
option.  A system may have several versions of diff (e.g., vendor's
and Gnu's).  Added codes to try them all until finding one that supports
it.  If found, use it; else stick with plain "diff" and may waste time
when there are no differences besides the ignorable patterns.

Platforms tested:
Solaris 2.7, Redhat 6.2