Pass rc.weekstart to libshared for ISO8601 weeknum parsing if "monday"

[smemsh]

Libshared master branch has been updated to have some consideration of Datetime::weekstart: date parsing (in particular week/day numbers) will behave differently depending on whether that value is 0 or 1 (see GothenburgBitFactory/libshared#81).

Previously, that code always behaved as the new code does when weekstart=0 when parsing dates (weekstart was used only for task calendar). The new code will use ISO8601 date parsing if that value is 1, which it is initialized to in the top of the file. Since this will cause a change in behavior, we can least surprise users by initializing it from user config, which is done in Context::StaticInitialization().

The hardcoded default rcfile sets weekstart to Sunday (0, the existing behavior) so only users that set rc.weekstart=monday explicitly will see new ISO8601 date parsing behavior, everyone else should see the same behavior as before.

Note: the week shortcuts (sow, sonw, etc) will still always use Monday weeks, regardless of weekstart, until the additional work mentioned in #3623 is done. That will be left for a part2 some time later. We can track that work in #3629.

Old code did not distinguish weekstart, and always used Sunday week parsing:

 $ task3.old rc.weekstart=sunday calc 2013-W49
2013-12-01T00:00:00

 $ task3.old rc.weekstart=monday calc 2013-W49
2013-12-01T00:00:00

The date 2013-12-01 is actually in week 48 for both Sunday and Monday-based weeks, and week 49 only begins on the 2nd, which is seen correctly in ISO-8601 weeks:

 $ date -d 2013-12-01 +s:%U,m:%V
s:48,m:48

 $ date -d 2013-12-02 +s:%U,m:%V
s:48,m:49

This is reflected in the new code when Monday is selected as the weekstart:

 $ src/task rc.weekstart=sunday calc 2013-W49
2013-12-01T00:00:00

 $ src/task rc.weekstart=monday calc 2013-W49
2013-12-02T00:00:00

 $ env - ncal -w -M 12 2013
    December 2013
Mo     2  9 16 23 30
Tu     3 10 17 24 31
We     4 11 18 25
Th     5 12 19 26
Fr     6 13 20 27
Sa     7 14 21 28
Su  1  8 15 22 29
   48 49 50 51 52  1

The old code is left intact when Sunday-based weeks are used. It seems to be Taskwarrior's own definition of weeks, since it differs from strftime() expansion %U, as noted above. Nonetheless, the Sunday behavior was left unchanged from previous Taskwarrior versions, and that remains the default. For correctness, it is recommended for users to configure Monday-based weeks, which would now be determined using an ISO-8601 standard algorithm.

Please apply, thanks.

1. Fixes Update libshared to get new weekstart #3623 (brings in new weekstart code from libshared)
2. Fixes ISO week starts on wrong day in task calc output #2922 (considers weekstart when parsing week numbers)
3. Unblocks Limit the allowed epoch timestamps #3651 (updates libshared peg to current master)
4. Enables 2.6.X: eow does not respect weekstart #3629 (part1 done for parsing; more work needed)

[smemsh]

Eventually we should add some tests for weekstart=1. The existing tests probably do not set weekstart except for the calendar, which was the only place weekstart was used before (internally in CmdCalendar only). So, weekstart=0 should be tested enough, but perhaps not weekstart=1.

However, note that I did duplicate all the tests under both weekstarts in libshared itself, and those all pass. So I will probably not add weekstart=1 tests into Taskwarrior until I look at part2, ie #3629.

[djmitche]

Thanks for the detailed work on this!