suboptimal behavior of fast-import in some cases with "from"

[Mike Hommey <mh@glandium.org>]

Hi,

I did something "stupid" with a script using fast-import, and that made
the whole process ~20% slower on Linux and 400~500% slower on Mac. The
reason this happened is that the script was essentially adding a "from"
to every "commit" command, even when the "from" commit is the current
head of the branch.

One of the first things parse_from does is unconditionally throw away
the tree for the given branch, and then the "from" tree is loaded. So
when the "from" commit is the current head of the branch, that make
fast-import do more work than necessary.

Even more so when the pack flush code in gfi_unpack_entry is triggered,
which, on mac, is extra slow (and explains the huge slow down there).

Now, I do understand that my script is doing something stupid. So the
question is whether it's worth fixing in fast-import or not. I already
fixed my script anyways.

Mike