git-pack-objects: make "--window=x" semantics more logical.

git-pack-objects: make "--window=x" semantics more logical.

A zero disables delta generation (like before), but we make the window
be one bigger than specified, since we use one entry for the one to be
tested (it used to be that "--window=1" was meaningless, since we'd have
used up the single-entry window with the entry to be tested, and had no
chance of actually ever finding a delta).

The default window remains at 10, but now it really means "test the 10
closest objects", not "test the 9 closest objects".