filter-repo: fix crazy timezone issues

Oh, boy, timezone +051800 exists in the wild.  Is that 0518 hours and 00
minutes?  Or 05 hours and 1800 minutes?  Or 051 hours and 800 minutes?
Attempt to do something sane with these broken commits that fast-import
barfs on.  Also, fix an old bug in the handling of ahead-of-UTC timezones.

Signed-off-by: Elijah Newren <newren@gmail.com>

git-filter-repo

@@ -53,9 +53,23 @@ class FixedTimeZone(tzinfo):
 
   def __init__(self, offset_string):
     tzinfo.__init__(self)
-    minus, hh, mm = re.match(r'^([-+]?)(\d\d)(\d\d)$', offset_string).groups()
-    sign = minus and -1 or 1
-    self._offset = timedelta(minutes = sign*(60*int(hh) + int(mm)))
+    try:
+      sign, hh, mm = re.match(r'^([-+]?)(\d\d)(\d\d)$', offset_string).groups()
+    except AttributeError:
+      # TimeZone idiocy; IST is any of four timezones, so someone translated
+      # it to something that was totally invalid...and it got recorded that
+      # way.  Others have suggested just using an invalid timezone that
+      # fast-import will not choke on.  Let's do that.  Note that +051800
+      # seems to be the only weird timezone found in the wild, by me or some
+      # other posts google returned on the subject...
+      if offset_string == '+051800':
+        sign, hh, mm = '+', '02', '61'
+        offset_string = offset_string.replace('+051800', '+0261')
+      else:
+        raise AttributeError("Could not parse {} as timezone"
+                             .format(offset_string))
+    factor = -1 if (sign and sign == '-') else 1
+    self._offset = timedelta(minutes = factor*(60*int(hh) + int(mm)))
     self._offset_string = offset_string
 
   def utcoffset(self, dt):