progress: create GIT_PROGRESS_DELAY

progress: create GIT_PROGRESS_DELAY

The start_delayed_progress() method is a preferred way to show
optional progress to users as it ignores steps that take less
than two seconds. However, this makes testing unreliable as tests
expect to be very fast.

In addition, users may want to decrease or increase this time
interval depending on their preferences for terminal noise.

Create the GIT_PROGRESS_DELAY environment variable to control
the delay set during start_delayed_progress(). Set the value
in some tests to guarantee their output remains consistent.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>