[infra] Change git_utils.NewGitRepo to use the default "git clone" dir

Change-Id: I616d2a119886d04e2b21499aece3e042226f6f3f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/375057
Reviewed-by: Ravi Mistry <rmistry@google.com>
Commit-Queue: Eric Boren <borenet@google.com>
1 file changed