t9300: work around partial read bug in Dash v0.5.13

When executing t9300 with Dash v0.5.13.1 we can see that the test hangs
completely with the following (condensed) trace:

  git fast-import
  + error=1
  + read output
  + cat input
  + echo checkpoint
  + echo progress checkpoint
  + test rogress checkpoint = progress checkpoint
  + test rogress checkpoint = UNEXPECTED
  + echo cruft: rogress checkpoint
  cruft: rogress checkpoint
  + read output
  + test  = progress checkpoint
  + test  = UNEXPECTED
  + echo cruft:
  cruft:
  + read output

Basically, what's happening here is that we spawn git-fast-import(1) and
wait for it to output a certain string, "progress checkpoint". Curiously
though, what we end up reading is "rogress checkpoint" -- so the first
byte of the expected string is missing.

Same as in the preceding commit, this seems to be a bug in Dash itself
that bisects to c5bf970 (expand: Add multi-byte support to pmatch,
2024-06-02). But other than in the preceding commit, this bug has
already been fixed upstream in 079059a (input: Fix heap-buffer-overflow
in preadbuffer on long lines, 2026-02-11), which is part of v0.5.13.2.

For now though, work around the bug by waiting for the expected output
in a different way. There is no good reason why one version should work
better than the other, but at least the new version doesn't exhibit the
bug. And, if you ask me, it's also slightly easier to read.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9300-fast-import.sh

@@ -3635,25 +3635,21 @@ background_import_then_checkpoint () {
 		echo "progress checkpoint"
 	) >&8 &
 
-	error=1 ;# assume the worst
-	while read output <&9
-	do
-		if test "$output" = "progress checkpoint"
-		then
-			error=0
-			break
-		elif test "$output" = "UNEXPECTED"
-		then
-			break
-		fi
-		# otherwise ignore cruft
-		echo >&2 "cruft: $output"
-	done
+	last=$(
+		while read output <&9
+		do
+			if test "$output" = "progress checkpoint" || test "$output" = "UNEXPECTED"
+			then
+				echo "$output"
+				break
+			else
+				# otherwise ignore cruft
+				echo >&2 "cruft: $output"
+			fi
+		done
+	)
 
-	if test $error -eq 1
-	then
-		false
-	fi
+	test "$last" = "progress checkpoint"
 }
 
 background_import_still_running () {