Following renames

  Hi,

  so, now that I've put up with the fuzzy rename autodetection (for
now), I'd like to make cg-log auto-follow renames and I'm wondering
about the best implementation (it seems that I won't do without core
Git cooperation). I think it should be possible to implement in a way
so that it has minimal performance impact and therefore I can have it
turned on by default.

  Now I'm using the notorious

	git-rev-list listoffiles | git-diff-tree --stdin

pipeline in cg-log, and I'm wondering about the best way to add rename detection there.

  In [1], Linus suggests a non-core solution. Unfortunately, it doesn't
fly - it requires at least two git-ls-tree calls per revision which
would bog things down awfully (to roughly half of the original speed).

  But even if git-rev-list reported disappearing files, Cogito would
have to do a lot of complicated bookkeeping in order to properly track
renames in parallel branches - for each 'head' commit at any point of
the history traversal, you need to record a separate set of interesting
files. It would also have to restart git-rev-list at any moment when a
rename happens on any of the head commits. Scales well not.

  An obvious solution would be to have git-diff-tree --follow which
updates its interesting path set based on seen renames, and now that
I've written about non-linear history, it's obvious that it's incorrect.
The other obvious way to go is then to add rename detection support to
git-rev-list, and it's less obvious that this is a dead end too - I
didn't inspect the code myself yet, but for now I trust Linus in [2]
(I didn't quite understand the argument, I guess I need to sleep on it).

  So, any thoughts about how to approach this? Either git-diff-tree
would have to be taught about the heads bookkeeping, or the git-rev-list
hurdles would have to be overcome, or we might have a
git-rev-rename-filter or something, but that feels quite redundant and
might meet with the same problems as git-rev-list.

  == References ==

  [1] Oct 21 <Pine.LNX.4.64.0510211814050.10477@g5.osdl.org>
  [2] Oct 22 <Pine.LNX.4.64.0510221251330.10477@g5.osdl.org>