merge(third_party/git): Merge squashed git subtree at v2.23.0

Merge commit '1b593e1ea4' as 'third_party/git'

third_party/git/Documentation/date-formats.txt

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+DATE FORMATS
+------------
+
+The `GIT_AUTHOR_DATE`, `GIT_COMMITTER_DATE` environment variables
+ifdef::git-commit[]
+and the `--date` option
+endif::git-commit[]
+support the following date formats:
+
+Git internal format::
+	It is `<unix timestamp> <time zone offset>`, where `<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`.
+
+RFC 2822::
+	The standard email format as described by RFC 2822, for example
+	`Thu, 07 Apr 2005 22:13:13 +0200`.
+
+ISO 8601::
+	Time and date specified by the ISO 8601 standard, for example
+	`2005-04-07T22:13:13`. The parser accepts a space instead of the
+	`T` character as well.
++
+NOTE: In addition, the date part is accepted in the following formats:
+`YYYY.MM.DD`, `MM/DD/YYYY` and `DD.MM.YYYY`.