adb: fix infinite loop when attempting to push to //foo.

dirname (on glibc, at least) preserves multiple leading slashes, and we
were looping until path != "/", which would lead to an infinite loop
when attempting to push to a path like //data/local/tmp.

Bug: http://b/141311284
Test: python -m unittest test_device.FileOperationsTest.test_push_multiple_slash_root
Change-Id: I182b3e89ef52579c716fdb525e9215f1fe822477