approxidate: make "specials" respect fixed day-of-month

The special approxidate time formats, "noon" and "tea" differ from
"12pm" and "5pm" by having the feature of wrapping to the previous day
if the current time is before those hours:

	now  -> 2026-05-13 11:00:00 +0000

	12pm -> 2026-05-13 12:00:00 +0000
	5pm  -> 2026-05-13 17:00:00 +0000

	noon -> 2026-05-12 12:00:00 +0000
	tea  -> 2026-05-12 17:00:00 +0000

However, that logic carries too far.  Even when the date is specified,
the behavior of the "specials" depends on the current time.  Assuming
the same time as above, we get:

	today at noon -> 2026-05-12 12:00:00 +0000 (should be 13 May)
	13 May at tea -> 2026-05-12 17:00:00 +0000

or, using an example mentioned in date-formats.adoc:

	last Friday at noon -> 2026-05-07 12:00:00 +0000 (should be 8 May)

The quirk seems to be rather old.  Already in 2006, Linus Torvalds
remarked that the date yielded by "one year ago yesterday at tea-time"
was "just silly and not even correct".  Indeed, even today it gives:

	One year ago yesterday at tea-time -> 2025-05-11 17:00:00 +0000
	  (should be 12 May)

Let's fix all of those with a simple patch.  Check whether we already
have a specified day-of-month in `tm->tm_mday` and make `date_time()`
stick to it.  Ensure the correct behavior with relevant tests.

Links:
  1. https://lore.kernel.org/git/Pine.LNX.4.64.0610101102560.3952@g5.osdl.org/

Signed-off-by: Tuomas Ahola <taahol@utu.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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