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

Review URL: https://codereview.chromium.org/561193004

Cr-Original-Commit-Position: refs/heads/master@{#294679}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 1c5731d3c4397e3ad41ae3c47089896d057a59c2
1 file changed