Reuse the same temp index in a transaction

Instead of making a new temp index every time we need one, just keep
reusing the same one. And keep track of which tree is currently stored
in it -- if we do several consecutive successful pushes, it's always
going to be the "right" tree so that we don't have to call read-tree
before each patch application.

The motivation behind this change is of course that it makes things
faster.

(The same simple test as in the previous patch -- pushing 250 patches
in a 32k-file repository, with one file-level merge necessary per push
-- went from 0.36 to 0.19 seconds per patch with this patch applied.)

Signed-off-by: Karl Hasselström <kha@treskal.com>