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-Commit-Position: refs/heads/master@{#294679}

third_party/yasm/yasm_assemble.gni

@@ -84,6 +84,7 @@ if (is_mac || is_ios) {
 # Default yasm include dirs. Make it match the native build (source root and
 # root generated code directory).
 _yasm_flags += [
+  "-I.",
   # Using "//." will produce a relative path "../.." which looks better than
   # "../../" which will result from using "//" as the base (although both
   # work). This is because rebase_path will terminate the result in a slash if

tools/gn/last_commit_position.py

@@ -93,7 +93,7 @@ git_directory = sys.argv[1]
 output_file = sys.argv[2]
 header_guard = sys.argv[3]
 
-value = FetchCommitPosition(".")
+value = FetchCommitPosition(git_directory)
 if not value:
   print "Could not get last commit position."
   sys.exit(1)