commit: accept more date formats for "--date"

Right now we pass off the string found by "--date" straight
to the fmt_ident function, which will use our strict
parse_date to normalize it. However, this means obvious
things like "--date=now" or "--date=2.days.ago" will not
work.

Instead, let's fallback to the approxidate function to
handle this for us. Note that we must try parse_date
ourselves first, even though approxidate will try strict
parsing itself. The reason is that approxidate throws away
any timezone information it sees from the strict parsing,
and we want to preserve it. So asking for:

  git commit --date="@1234567890 -0700"

continues to set the date in -0700, regardless of what the
local timezone is.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/commit.c

@@ -526,10 +526,29 @@ static int sane_ident_split(struct ident_split *person)
 	return 1;
 }
 
+static int parse_force_date(const char *in, char *out, int len)
+{
+	if (len < 1)
+		return -1;
+	*out++ = '@';
+	len--;
+
+	if (parse_date(in, out, len) < 0) {
+		int errors = 0;
+		unsigned long t = approxidate_careful(in, &errors);
+		if (errors)
+			return -1;
+		snprintf(out, len, "%lu", t);
+	}
+
+	return 0;
+}
+
 static void determine_author_info(struct strbuf *author_ident)
 {
 	char *name, *email, *date;
 	struct ident_split author;
+	char date_buf[64];
 
 	name = getenv("GIT_AUTHOR_NAME");
 	email = getenv("GIT_AUTHOR_EMAIL");
@@ -574,8 +593,12 @@ static void determine_author_info(struct strbuf *author_ident)
 		email = xstrndup(lb + 2, rb - (lb + 2));
 	}
 
-	if (force_date)
-		date = force_date;
+	if (force_date) {
+		if (parse_force_date(force_date, date_buf, sizeof(date_buf)))
+			die(_("invalid date format: %s"), force_date);
+		date = date_buf;
+	}
+
 	strbuf_addstr(author_ident, fmt_ident(name, email, date, IDENT_STRICT));
 	if (!split_ident_line(&author, author_ident->buf, author_ident->len) &&
 	    sane_ident_split(&author)) {

t/t7501-commit.sh

@@ -351,8 +351,16 @@ test_expect_success 'commit mentions forced date in output' '
 	grep "Date: *Sat Jan 2 03:04:05 2010" output
 '
 
-test_expect_success 'commit complains about bogus date' '
-	test_must_fail git commit --amend --date=10.11.2010
+test_expect_success 'commit complains about completely bogus dates' '
+	test_must_fail git commit --amend --date=seventeen
+'
+
+test_expect_success 'commit --date allows approxidate' '
+	git commit --amend \
+		--date="midnight the 12th of october, anno domini 1979" &&
+	echo "Fri Oct 12 00:00:00 1979 +0000" >expect &&
+	git log -1 --format=%ad >actual &&
+	test_cmp expect actual
 '
 
 test_expect_success 'sign off (1)' '