Make for-each-ref's grab_date() support per-atom formatting

grab_date() gets an extra parameter - atomname; this extra parameter is
checked to see if it has a ":<format>" extra component in it, and if so
that "<format>" string is passed to parse_date_format() to produce an
enum date_mode value which is then further passed to show_date().

In short it allows the user of git-for-each-ref to do things like this:

 $ git-for-each-ref --format='%(taggerdate:default)' refs/tags/v1.5.2
 Sun May 20 00:30:42 2007 -0700
 $ git-for-each-ref --format='%(taggerdate:relative)' refs/tags/v1.5.2
 4 months ago
 $ git-for-each-ref --format='%(taggerdate:short)' refs/tags/v1.5.2
 2007-05-20
 $ git-for-each-ref --format='%(taggerdate:local)' refs/tags/v1.5.2
 Sun May 20 08:30:42 2007
 $ git-for-each-ref --format='%(taggerdate:iso8601)' refs/tags/v1.5.2
 2007-05-20 00:30:42 -0700
 $ git-for-each-ref --format='%(taggerdate:rfc2822)' refs/tags/v1.5.2
 Sun, 20 May 2007 00:30:42 -0700

The default, when no ":<format>" is specified is ":default", leaving the
existing behaviour unchanged.

Signed-off-by: Andy Parkins <andyparkins@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-for-each-ref.txt

@@ -100,6 +100,11 @@ In any case, a field name that refers to a field inapplicable to
 the object referred by the ref does not cause an error.  It
 returns an empty string instead.
 
+As a special case for the date-type fields, you may specify a format for
+the date by adding one of `:default`, `:relative`, `:short`, `:local`,
+`:iso8601` or `:rfc2822` to the end of the fieldname; e.g.
+`%(taggerdate:relative)`.
+
 
 EXAMPLES
 --------

builtin-for-each-ref.c

@@ -356,12 +356,26 @@ static const char *copy_email(const char *buf)
 	return line;
 }
 
-static void grab_date(const char *buf, struct atom_value *v)
+static void grab_date(const char *buf, struct atom_value *v, const char *atomname)
 {
 	const char *eoemail = strstr(buf, "> ");
 	char *zone;
 	unsigned long timestamp;
 	long tz;
+	enum date_mode date_mode = DATE_NORMAL;
+	const char *formatp;
+
+	/*
+	 * We got here because atomname ends in "date" or "date<something>";
+	 * it's not possible that <something> is not ":<format>" because
+	 * parse_atom() wouldn't have allowed it, so we can assume that no
+	 * ":" means no format is specified, and use the default.
+	 */
+	formatp = strchr(atomname, ':');
+	if (formatp != NULL) {
+		formatp++;
+		date_mode = parse_date_format(formatp);
+	}
 
 	if (!eoemail)
 		goto bad;
@@ -371,7 +385,7 @@ static void grab_date(const char *buf, struct atom_value *v)
 	tz = strtol(zone, NULL, 10);
 	if ((tz == LONG_MIN || tz == LONG_MAX) && errno == ERANGE)
 		goto bad;
-	v->s = xstrdup(show_date(timestamp, tz, 0));
+	v->s = xstrdup(show_date(timestamp, tz, date_mode));
 	v->ul = timestamp;
 	return;
  bad:
@@ -398,7 +412,7 @@ static void grab_person(const char *who, struct atom_value *val, int deref, stru
 		if (name[wholen] != 0 &&
 		    strcmp(name + wholen, "name") &&
 		    strcmp(name + wholen, "email") &&
-		    strcmp(name + wholen, "date"))
+		    prefixcmp(name + wholen, "date"))
 			continue;
 		if (!wholine)
 			wholine = find_wholine(who, wholen, buf, sz);
@@ -410,8 +424,8 @@ static void grab_person(const char *who, struct atom_value *val, int deref, stru
 			v->s = copy_name(wholine);
 		else if (!strcmp(name + wholen, "email"))
 			v->s = copy_email(wholine);
-		else if (!strcmp(name + wholen, "date"))
-			grab_date(wholine, v);
+		else if (!prefixcmp(name + wholen, "date"))
+			grab_date(wholine, v, name);
 	}
 
 	/* For a tag or a commit object, if "creator" or "creatordate" is
@@ -431,8 +445,8 @@ static void grab_person(const char *who, struct atom_value *val, int deref, stru
 		if (deref)
 			name++;
 
-		if (!strcmp(name, "creatordate"))
-			grab_date(wholine, v);
+		if (!prefixcmp(name, "creatordate"))
+			grab_date(wholine, v, name);
 		else if (!strcmp(name, "creator"))
 			v->s = copy_line(wholine);
 	}