Player's gravity is calculated twice on the tick they activate their elytra, causing lower jump height

The player's gravity is calculated twice on the tick they activate their elytra, causing jump height lower. This happens since 15w41a.

Steps to reproduce:

1. Equip an elytra and switch to survival mode.
2. Run /tick rate 1 (this happens on normal tick rate, but on low tick rates it can be seen clearly).
3. Open the F3 debug screen, face directly down, jump and activate your elytra 2 ticks after you jump (your height should be 0.7532 blocks above ground 2 ticks after you jump).
4. Observe your maximum height.
5. After you land, jump again, don't activate your elytra.
6. Observe your maximum height.

Expected result:
Maximum height in step 4 and step 6 are same. (when you face directly down, the elytra won't reduce your gravity and you should be free falling)

Actual result:
Maximum height in step 4 and step 6 are different. In step 4, the height is 1.00407 blocks. In step 6, the height is 1.25220 blocks.

Analysis:
On the first tick you activate your elytra, your gravity is calculated twice, cause your upward speed to be lower than normal (it has lowered from 0.3332b/t to 0.16477b/t, and normally it is from 0.3332 b/t to 0.24814b/t, so your jump height is lower).