Features:

- Non-verbose output is now limited to a one-line progress
indicator.

- Verbose output is now collated per subprocess.  As soon as a
subprocess finishes, its full output is dumped to terminal.

- Verbose output is prefixed with timestamps representing elapsed
time since the beginning of the gclient invocation.

- git progress indicators ("Receiving objects", etc.) are limited to
one line every 10 seconds.

- In both verbose and non-verbose mode, if a failure occurs, the
full output of the failed update operation is dumped to terminal
just before exit.

- In the event that updates are progressing, but slowly,
"Still working" messages will be printed periodically, to pacify
users and buildbots.

BUG=
R=hinoka@google.com

Review URL: https://codereview.chromium.org/227163002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@262500 0039d316-1c4b-4281-b951-d872f2087c98