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>

progress.c

@@ -14,6 +14,7 @@
 #include "strbuf.h"
 #include "trace.h"
 #include "utf8.h"
+#include "config.h"
 
 #define TP_IDX_MAX      8
 
@@ -267,9 +268,19 @@ static struct progress *start_progress_delay(const char *title, uint64_t total,
 	return progress;
 }
 
+static int get_default_delay(void)
+{
+	static int delay_in_secs = -1;
+
+	if (delay_in_secs < 0)
+		delay_in_secs = git_env_ulong("GIT_PROGRESS_DELAY", 2);
+
+	return delay_in_secs;
+}
+
 struct progress *start_delayed_progress(const char *title, uint64_t total)
 {
-	return start_progress_delay(title, total, 2, 0);
+	return start_progress_delay(title, total, get_default_delay(), 0);
 }
 
 struct progress *start_progress(const char *title, uint64_t total)
@@ -294,7 +305,7 @@ struct progress *start_sparse_progress(const char *title, uint64_t total)
 struct progress *start_delayed_sparse_progress(const char *title,
 					       uint64_t total)
 {
-	return start_progress_delay(title, total, 2, 1);
+	return start_progress_delay(title, total, get_default_delay(), 1);
 }
 
 static void finish_if_sparse(struct progress *progress)