[PATCH 0 of 1] compare grep result between target and its parent

[FUJIWARA Katsunori]

Sometime 'hg grep --all' seems to show unexpected result.

I found that typical case is that grep target is added at (*) revision
in the tree shown below.

     +--- 1(*) --- 3
     0 
     +--- 2 ------ 4

Now, I expect 'hg grep --all' to show only rev:1 which is first
appearance of target line.

But 'hg grep --all' will tell:

  target line dis-appeared at 3 => 4
  target line     appeared at 2 => 3
  target line dis-appeared at 1 => 2
  target line     appeared at 0 => 1

because current 'hg grep' implementation compares not between target
revision and its parent, but between neighbor revisions in walkthrough
order.

So, I wrote experimental patch to adjust this behavior, and please
comment on it. 

Especially, I want to know wether there are any reasons to choose
current implementation: performance, reduction of memory consumption,
curtural one, and so on

I checked performance of this patch by "hg grep --follow --all
walkchangerevs" on whole Mercurial repo, and patched version could
complete as fast as un-patched one.

# memory consumption is not checked yet, sorry and I'll do