GN: fail gracefully when git commit info cannot be extracted

See https://groups.google.com/a/chromium.org/d/msg/gn-dev/7nlJv486bD4/V9ZsfV_nBAAJ
for context.

BUG=551343
R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/2237913002 .

Cr-Commit-Position: refs/heads/master@{#412213}
1 file changed