diff: fix a bug in diffing against stdin. Closes 7784

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

editors/diff.c

@@ -740,9 +740,10 @@ static int diffreg(char *file[2])
 			unlink(name);
 			if (bb_copyfd_eof(fd, fd_tmp) < 0)
 				xfunc_die();
-			if (fd) /* Prevents closing of stdin */
+			if (fd != STDIN_FILENO)
 				close(fd);
 			fd = fd_tmp;
+			xlseek(fd, 0, SEEK_SET);
 		}
 		fp[i] = fdopen(fd, "r");
 	}

testsuite/diff.tests

@@ -44,6 +44,17 @@ testing "diff of stdin, twice" \
 	"" \
 	"stdin"
 
+testing "diff of empty file against stdin" \
+	"diff -u - input | $TRIM_TAB" \
+"\
+--- -
++++ input
+@@ -1 +0,0 @@
+-a
+" \
+	"" \
+	"a\n"
+
 testing "diff of empty file against nonempty one" \
 	"diff -u - input | $TRIM_TAB" \
 "\