If a hook prints error/warning output that's color-coded, gclient
will cause the coloring to be disabled since those hooks aren't
called directly from a terminal.

By emulating a terminal when launching subprocs from gclient, we can
convince them to keep the color escape codes.

LED builds with both //third_party/depot_tools rolled to this CL, as
well as depot_tools in the recipe bundle rolled to this CL:
linux: https://ci.chromium.org/swarming/task/4e40237985888310
mac: https://ci.chromium.org/swarming/task/4e4023ea0c829710
win: https://ci.chromium.org/swarming/task/4e4024612e03dc10

Bug: 1034063
Change-Id: I4150f66ef215ece06f4c32482dcd4ded14eb1bfe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2368435Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Ben Pastene <bpastene@chromium.org>