util.datestr: do not crash on revisions with negative timestamp (issue2513)

Python's time.gmtime(lt) fails on Windows, producing a traceback with

  ValueError: (22, 'Invalid argument')

if lt < -43200.

We get a local time boundary value of -43200 if we take "the epoch"

   Thu Jan 01 00:00:00 1970 = time.gmtime(0)

from timezone 'UTC+0' into timezone 'UTC-12'. All other timezones will have
larger local time values for that point in time.

Aborting with a traceback on 'hg log' for revisions with a timestamp value
< -43200 is clearly not acceptable.

Returning "invalid timestamp" or similar as string representation is not an
option either, since that may crash other tools which parse the output of
'hg log'.

Instead, we teach util.datestr() to return the epoch in timezone UTC+0 on
*all platforms*, represented by the string

  Thu Jan 01 00:00:00 1970 +0000

if the timestamp's unix time value is negative.

(based on a patch originally proposed by Benjamin Pollack)

mercurial/util.py

@@ -1023,6 +1023,9 b" def datestr(date=None, format='%a %b %d "
     number of seconds away from UTC. if timezone is false, do not
     append time zone to string."""
     t, tz = date or makedate()
+    if t < 0:
+        t = 0   # time.gmtime(lt) fails on Windows for lt < -43200
+        tz = 0
     if "%1" in format or "%2" in format:
         sign = (tz > 0) and "-" or "+"
         minutes = abs(tz) // 60