Handling renames.

[David Woodhouse <dwmw2@infradead.org>]

I've been looking at tracking file revisions. One proposed solution was
to have a separate revision history for individual files, with a new
kind of 'filecommit' object which parallels the existing 'commit',
referencing a blob instead of a tree. Then trees would reference such
objects instead of referencing blobs directly.

I think that introduces a lot of redundancy though, because 99% of the
time, the revision history of the individual file is entirely
reproducible from the revision history of the tree. It's only when files
are renamed that we fall over -- and I think we can handle renames
fairly well if we just log them in the commit object. 

My 'gitfilelog.sh' script is already capable of tracking a given file
back through multiple tree commits, listing those commits where the file
in question was actually changed. It uses my patched version of diff-
tree which supports 'diff-tree <TREE_A> <TREE_B> <filename>' in order to
do this.

By storing rename information in the commit object, the script (or a
reimplementation of a similar algorithm) could know when to change the
filename it's looking for, as it goes back through the tree. That ought
to be perfectly sufficient.

So a commit involving a rename would look something like this...

	tree 82ba574c85e9a2e4652419c88244e9dd1bfa8baa
	parent bb95843a5a0f397270819462812735ee29796fb4
	rename foo.c bar.c
	author David Woodhouse <dwmw2@hades.cambridge.redhat.com> 1113499881 +0100
	committer David Woodhouse <dwmw2@hades.cambridge.redhat.com> 1113499881 +0100
	Rename foo.c to bar.c and s/foo_/bar_/g

Opinions? Dissent? We'd probably need to escape the filenames in some
way -- handwave over that for now.

-- 
dwmw2