revset: add desc(string) to search in commit messages Like keyword(), but does not search in filenames and users. No grepdesc() or descgrep() added, because it might be bad to introduce grepfoo() versions of too many string searches.

--- a/tests/test-revset.t	Thu Jun 16 22:03:26 2011 +0200
+++ b/tests/test-revset.t	Thu Jun 16 22:47:34 2011 +0200
@@ -190,6 +190,8 @@
   1
   3
   5
+  $ log 'desc(B)'
+  5
   $ log 'descendants(2 or 3)'
   2
   3