Ok, if you guys have a current git source, and want to try something out,
this fairly small patch does this.
As mentioned, git already supports the notion of "try to break files with
the same name if the contents are too dissimilar". In other words, even if
a file exists under the same name in both the old revision and in the
newer one, we'll look at just how big the changes are, and if git decides
that it looks like the whole file was rewritten, then git will split up
the diff into a "delete old contents" and "create new contents". That then
allows it to consider the file for rename detection.
(The rename detection may, of course, decide that the original file was
the best source after all.. ;)
However, "git log --follow" didn't ever actually enable that for the logic
that tries to figure out where a file came from, so you would only see
this when generating the diffs, never in the file history logic. That's
an easy one-liner to tree-diff.c: try_to_follow_renames().
However, when I actually tried it, it turns out that the break logic was
broken - nobody has ever really depended on it. So while it was a
one-liner to make "git log --follow" understand to break files that seem
to be totally rewritten, it still didn't actually work, because the
changes that totally rewrote vmlinux.lds.S wouldn't trigger the break
logic.
So most of this (still fairly small patch) is just fixing the break logic
in diffcore-break.c:should_break().
It hasn't gotten a lot of testing, but it does actually improve other
cases too, so I think this is the right thign to do. I'll bring it up on
the git lists.
Oh, and with this patch, the "break same filename" is still off by
default. You need to do
git log --follow -B arch/x86/kernel/vmlinux_64.lds.S
to enable the file-rewritten-so-break-associations detection. I suspect
it makes sense to enable -B by default when using --follow (--follow
already obviously implies rename detection), but that's a separate and ...
