tests: use pdiff instead of diff for portability

"diff" command might cause redundant message, "No differences
encountered" on Solaris for example. But suppressing option like "-q"
isn't portable, because POSIX specification doesn't define it.

pdiff script was introduced by b59ef0c21405 to stabilize output of
standard diff command on each platforms.

tests/test-cache-abuse.t

@@ -1,97 +1,97
 Enable obsolete markers
 
   $ cat >> $HGRCPATH << EOF
   > [experimental]
   > evolution=createmarkers
   > [phases]
   > publish=False
   > EOF
 
 Build a repo with some cacheable bits:
 
   $ hg init a
   $ cd a
 
   $ echo a > a
   $ hg ci -qAm0
   $ hg tag t1
   $ hg book -i bk1
 
   $ hg branch -q b2
   $ hg ci -Am1
   $ hg tag t2
 
   $ echo dumb > dumb
   $ hg ci -qAmdumb
   $ hg debugobsolete b1174d11b69e63cb0c5726621a43c859f0858d7f
   obsoleted 1 changesets
 
   $ hg phase -pr t1
   $ hg phase -fsr t2
 
 Make a helper function to check cache damage invariants:
 
 - command output shouldn't change
 - cache should be present after first use
 - corruption/repair should be silent (no exceptions or warnings)
 - cache should survive deletion, overwrite, and append
 - unreadable / unwriteable caches should be ignored
 - cache should be rebuilt after corruption
 
   $ damage() {
   >  CMD=$1
   >  CACHE=.hg/cache/$2
   >  CLEAN=$3
   >  hg $CMD > before
   >  test -f $CACHE || echo "not present"
   >  echo bad > $CACHE
   >  test -z "$CLEAN" || $CLEAN
   >  hg $CMD > after
-  >  diff -u before after || echo "*** overwrite corruption"
+  >  "$RUNTESTDIR/pdiff" before after || echo "*** overwrite corruption"
   >  echo corruption >> $CACHE
   >  test -z "$CLEAN" || $CLEAN
   >  hg $CMD > after
-  >  diff -u before after || echo "*** append corruption"
+  >  "$RUNTESTDIR/pdiff" before after || echo "*** append corruption"
   >  rm $CACHE
   >  mkdir $CACHE
   >  test -z "$CLEAN" || $CLEAN
   >  hg $CMD > after
-  >  diff -u before after || echo "*** read-only corruption"
+  >  "$RUNTESTDIR/pdiff" before after || echo "*** read-only corruption"
   >  test -d $CACHE || echo "*** directory clobbered"
   >  rmdir $CACHE
   >  test -z "$CLEAN" || $CLEAN
   >  hg $CMD > after
-  >  diff -u before after || echo "*** missing corruption"
+  >  "$RUNTESTDIR/pdiff" before after || echo "*** missing corruption"
   >  test -f $CACHE || echo "not rebuilt"
   > }
 
 Beat up tags caches:
 
   $ damage "tags --hidden" tags2
   $ damage tags tags2-visible
   $ damage "tag -f t3" hgtagsfnodes1
 
 Beat up branch caches:
 
   $ damage branches branch2-base "rm .hg/cache/branch2-[vs]*"
   $ damage branches branch2-served "rm .hg/cache/branch2-[bv]*"
   $ damage branches branch2-visible
   $ damage "log -r branch(.)" rbc-names-v1
   $ damage "log -r branch(default)" rbc-names-v1
   $ damage "log -r branch(b2)" rbc-revs-v1
 
 We currently can't detect an rbc cache with unknown names:
 
   $ damage "log -qr branch(b2)" rbc-names-v1
   --- before	* (glob)
   +++ after	* (glob)
   @@ -1,8 +?,0 @@ (glob)
   -2:5fb7d38b9dc4
   -3:60b597ffdafa
   -4:b1174d11b69e
   -5:6354685872c0
   -6:5ebc725f1bef
   -7:7b76eec2f273
   -8:ef3428d9d644
   -9:ba7a936bc03c
   *** append corruption