revert 0-formatting to be compatible with previous behaviour

as we format differences (growing/shrinking) we want:
-N  # negative / shrinking
 0  # no change -> no +/- sign here!
+N  # positive / growing

src/borg/helpers/parseformat.py

@@ -271,7 +271,7 @@ def parse_file_size(s):
 
 
 def sizeof_fmt(num, suffix='B', units=None, power=None, sep='', precision=2, sign=False):
-    sign = '+' if sign else ''
+    sign = '+' if sign and num > 0 else ''
     fmt = '{0:{1}.{2}f}{3}{4}{5}'
     prec = 0
     for unit in units[:-1]:

src/borg/testsuite/helpers.py

@@ -579,7 +579,7 @@ def test_file_size_precision():
 
 def test_file_size_sign():
     si_size_map = {
-        0: '+0 B',
+        0: '0 B',
         1: '+1 B',
         1234: '+1.23 kB',
         -1: '-1 B',