doc: document and test @ prefix for raw timestamps

The Git internal date format `<unix-timestamp> <time-zone-offset>`
fails to parse when the timestamp is less than 100,000,000 (fewer than
9 digits). This happens to avoid potential ambiguity with other date
formats such as `YYYYMMDD`, especially when used with approxidate.

To force the parser to interpret the value as a raw timestamp, it must
be prefixed with `@` (e.g., `@0 +0000`). This behavior was introduced
in 2c733fb (parse_date(): '@' prefix
forces git-timestamp, 2012-02-02) but was never documented.

Document the `@` prefix in `Documentation/date-formats.adoc` to make
this behavior explicit. Also add test cases to `t/t0006-date.sh` to
verify and demonstrate the difference between prefixed and unprefixed
small timestamps (e.g., `@2000` vs `2000`).

Signed-off-by: Luna Schwalbe <dev@luna.gl>
Co-authored-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: lunagl

Documentation/date-formats.adoc

@@ -9,6 +9,11 @@ Git internal format::
 	`<unix-timestamp>` is the number of seconds since the UNIX epoch.
 	`<time-zone-offset>` is a positive or negative offset from UTC.
 	For example CET (which is 1 hour ahead of UTC) is `+0100`.
++
+It is safer to prepend the `<unix-timestamp>` with `@` (e.g.,
+`@0 +0000`), which forces Git to interpret it as a raw timestamp. This
+is required for values less than 100,000,000 (which have fewer than 9
+digits) to avoid confusion with other date formats like `YYYYMMDD`.
 
 RFC 2822::
 	The standard date format as described by RFC 2822, for example

t/t0006-date.sh

@@ -138,6 +138,13 @@ check_parse '1969-12-31 23:59:59 Z' bad
 check_parse '1969-12-31 23:59:59 +11' bad
 check_parse '1969-12-31 23:59:59 -11' bad
 
+# pathologically small timestamps requiring `@` prefix
+check_parse '@0 +0000' '1970-01-01 00:00:00 +0000'
+check_parse '@99999999 +0000' '1973-03-03 09:46:39 +0000'
+check_parse '99999999 +0000' bad
+check_parse '@100000000 +0000' '1973-03-03 09:46:40 +0000'
+check_parse '100000000 +0000' '1973-03-03 09:46:40 +0000'
+
 REQUIRE_64BIT_TIME=HAVE_64BIT_TIME
 check_parse '2099-12-31 23:59:59' '2099-12-31 23:59:59 +0000'
 check_parse '2099-12-31 23:59:59 +00' '2099-12-31 23:59:59 +0000'
@@ -195,6 +202,10 @@ check_approxidate '6AM, June 7, 2009' '2009-06-07 06:00:00'
 check_approxidate '2008-12-01' '2008-12-01 19:20:00'
 check_approxidate '2009-12-01' '2009-12-01 19:20:00'
 
+# ambiguous raw timestamp
+check_approxidate '2000 +0000' '2000-08-30 19:20:00'
+check_approxidate '@2000 +0000' '1970-01-01 00:33:20'
+
 check_date_format_human() {
 	t=$(($GIT_TEST_DATE_NOW - $1))
 	echo "$t -> $2" >expect