tests: use "hg help revisions.<predicate>" instead of grepping

We have specific syntax for displaying the help text for a particular
revset predicate, so use that instead of grepping through the full
output.

tests/test-mq.t

@@ -888,9 +888,10 b' mq revset'
   $ hg log -r 'mq()' --template '{rev}\n'
   1
   2
-  $ hg help revsets | grep -i mq
+  $ hg help revisions.mq
       "mq()"
         Changesets managed by MQ.
+  
 
 bad node in status
 

tests/test-transplant.t

@@ -102,9 +102,10 b' test transplanted revset'
   7  b3
   $ hg log -r 'transplanted(head())' --template '{rev} {parents} {desc}\n'
   7  b3
-  $ hg help revsets | grep transplanted
+  $ hg help revisions.transplanted
       "transplanted([set])"
         Transplanted changesets in set, or all transplanted changesets.
+  
 
 test transplanted keyword