gn: fix last_commit_position.py and yasm_assemble.gni

gn: fix last_commit_position.py and yasm_assemble.gni

1- use the correct directory for git commands
2- yasm needs . in the include search path, otherwise I get:
FAILED: python ../../../third_party/yasm/run_yasm.py ./yasm -DPIC -felf64 -m amd64 -I../../.. -Igen -DARCH_X86_64 -w -P ../../../third_party/ffmpeg/chromium/config/Chromium/linux/x64/config.asm -I../../../third_party/ffmpeg/chromium/config/Chromium/linux/x64 -I../../../third_party/ffmpeg/libavcodec/x86 -I../../../third_party/ffmpeg/libavutil/x86 -I../../../third_party/ffmpeg -DPIC -o obj/third_party/ffmpeg/vp8dsp.o ../../../third_party/ffmpeg/libavcodec/x86/vp8dsp.asm
yasm: FATAL: unable to open include file `../../../third_party/ffmpeg/chromium/config/Chromium/linux/x64/config.asm'

BUG=None

Committed: https://crrev.com/1c5731d3c4397e3ad41ae3c47089896d057a59c2
Cr-Commit-Position: refs/heads/master@{#294679}

[piman, 2014-09-12 04:14:52]

piman@chromium.org changed reviewers:
+ brettw@chromium.org

[piman, 2014-09-12 04:14:52]

Not sure why other people don't run into the yasm issue. I think it's because my
outdir is 3 deep instead of 2, and I think the includes are tested against all
the other include paths (one of which is 2-deep relative to the root).

[brettw, 2014-09-12 21:25:17]

LGTM

[piman, 2014-09-12 21:38:07]

The CQ bit was checked by piman@chromium.org

[commit-bot: I haz the power, 2014-09-12 21:38:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/561193004/1

[commit-bot: I haz the power, 2014-09-12 22:17:10]

Committed patchset #1 (id:1) as 10fb747ffdb0672663a94cccaa7a4fa5c5055132

[commit-bot: I haz the power, 2014-09-12 22:20:48]

Patchset 1 (id:??) landed as
https://crrev.com/1c5731d3c4397e3ad41ae3c47089896d057a59c2
Cr-Commit-Position: refs/heads/master@{#294679}