[PATCH 5/8] progress.c: stop eagerly fflush(stderr) when not a terminal

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

It's the clear intention of the combination of 137a0d0ef56 (Flush
progress message buffer in display()., 2007-11-19) and
85cb8906f0e (progress: no progress in background, 2015-04-13) to call
fflush(stderr) when we have a stderr in the foreground, but we ended
up always calling fflush(stderr) seemingly by omission. Let's not.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 progress.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/progress.c b/progress.c
index 7fcc513717..1fade5808d 100644
--- a/progress.c
+++ b/progress.c
@@ -91,7 +91,8 @@ static void display(struct progress *progress, uint64_t n, const char *done)
 	}
 
 	if (show_update) {
-		if (is_foreground_fd(fileno(stderr)) || done) {
+		int stderr_is_foreground_fd = is_foreground_fd(fileno(stderr));
+		if (stderr_is_foreground_fd || done) {
 			const char *eol = done ? done : "\r";
 			size_t clear_len = counters_sb->len < last_count_len ?
 					last_count_len - counters_sb->len + 1 :
@@ -115,7 +116,8 @@ static void display(struct progress *progress, uint64_t n, const char *done)
 				fprintf(stderr, "%s: %s%*s", progress->title,
 					counters_sb->buf, (int) clear_len, eol);
 			}
-			fflush(stderr);
+			if (stderr_is_foreground_fd)
+				fflush(stderr);
 		}
 		progress_update = 0;
 	}
-- 
2.32.0.957.gd9e39d72fe6