Speed up hg grep by avoiding useless manifest parsing

In the kernel repo (tip = 2b89f7111b96), a "hg grep mpm MAINTAINERS" goes
from ~165s to 0.7s.  This could get even a bit faster if we broke out of
the loop after the first match, but I'm not sure how that would interact
with the --follow code.

This is obviously an extreme example, but other cases should also benefit
from this patch.

mercurial/commands.py

@@ -1072,19 +1072,19 b' def grep(ui, repo, pattern, *pats, **opt'
         if st == 'window':
             matches.clear()
         elif st == 'add':
-            mf = repo.changectx(rev).manifest()
+            ctx = repo.changectx(rev)
             matches[rev] = {}
             for fn in fns:
                 if fn in skip:
                     continue
                 try:
-                    grepbody(fn, rev, getfile(fn).read(mf[fn]))
+                    grepbody(fn, rev, getfile(fn).read(ctx.filenode(fn)))
                     fstate.setdefault(fn, [])
                     if follow:
-                        copied = getfile(fn).renamed(mf[fn])
+                        copied = getfile(fn).renamed(ctx.filenode(fn))
                         if copied:
                             copies.setdefault(rev, {})[fn] = copied[0]
-                except KeyError:
+                except revlog.LookupError:
                     pass
         elif st == 'iter':
             states = matches[rev].items()