check-code: add rule to detect usage of external diff via extdiff This rule detects "hg extdiff" invocation without -p/--program and -o/--option. This patch specifies "-p diff" explicitly in test-extdiff.t to avoid false positive matching.

tests: use portable diff script via extdiff extension Before this patch, some tests using external "diff" command via extdiff extension fail on Solaris, because of incompatibility of "diff" command and its output. For example, system standard "diff" (= /usr/bin/diff) on Solaris differs from GNU diff in points below: - "-N" (treat absent files as empty) option isn't supported - files are examined not in dictionary order (maybe, in order in storage) This patch introduces portable diff script "pdiff" and make tests use it via extdiff extension. For portability of tests, this patch invokes "pdiff" script with explicit "sh", because standard shell of runtime platform ("cmd.exe" on Windows) is used at first to invoke external diff command.