Move git cache_dir into gclient spec string.
Move git cache_dir into gclient spec string.

This will let us actually start using the git cache_dir. Previously, when it was
simply appended to the command as --cache_dir=, it would get ignored or
overridden since the command also contained --spec=.

R=iannucci@chromium.org, maruel@chromium.org
BUG=295119

Review URL: https://codereview.chromium.org/44763002

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@231164 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build