convert "enum date_mode" into a struct

In preparation for adding date modes that may carry extra
information beyond the mode itself, this patch converts the
date_mode enum into a struct.

Most of the conversion is fairly straightforward; we pass
the struct as a pointer and dereference the type field where
necessary. Locations that declare a date_mode can use a "{}"
constructor.  However, the tricky case is where we use the
enum labels as constants, like:

  show_date(t, tz, DATE_NORMAL);

Ideally we could say:

  show_date(t, tz, &{ DATE_NORMAL });

but of course C does not allow that. Likewise, we cannot
cast the constant to a struct, because we need to pass an
actual address. Our options are basically:

  1. Manually add a "struct date_mode d = { DATE_NORMAL }"
     definition to each caller, and pass "&d". This makes
     the callers uglier, because they sometimes do not even
     have their own scope (e.g., they are inside a switch
     statement).

  2. Provide a pre-made global "date_normal" struct that can
     be passed by address. We'd also need "date_rfc2822",
     "date_iso8601", and so forth. But at least the ugliness
     is defined in one place.

  3. Provide a wrapper that generates the correct struct on
     the fly. The big downside is that we end up pointing to
     a single global, which makes our wrapper non-reentrant.
     But show_date is already not reentrant, so it does not
     matter.

This patch implements 3, along with a minor macro to keep
the size of the callers sane.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pretty.c

@@ -399,7 +399,7 @@ static void add_rfc2047(struct strbuf *sb, const char *line, size_t len,
 }
 
 const char *show_ident_date(const struct ident_split *ident,
-			    enum date_mode mode)
+			    const struct date_mode *mode)
 {
 	unsigned long date = 0;
 	long tz = 0;
@@ -489,15 +489,15 @@ void pp_user_info(struct pretty_print_context *pp,
 	switch (pp->fmt) {
 	case CMIT_FMT_MEDIUM:
 		strbuf_addf(sb, "Date:   %s\n",
-			    show_ident_date(&ident, pp->date_mode));
+			    show_ident_date(&ident, &pp->date_mode));
 		break;
 	case CMIT_FMT_EMAIL:
 		strbuf_addf(sb, "Date: %s\n",
-			    show_ident_date(&ident, DATE_RFC2822));
+			    show_ident_date(&ident, DATE_MODE(RFC2822)));
 		break;
 	case CMIT_FMT_FULLER:
 		strbuf_addf(sb, "%sDate: %s\n", what,
-			    show_ident_date(&ident, pp->date_mode));
+			    show_ident_date(&ident, &pp->date_mode));
 		break;
 	default:
 		/* notin' */
@@ -671,7 +671,8 @@ static int mailmap_name(const char **email, size_t *email_len,
 }
 
 static size_t format_person_part(struct strbuf *sb, char part,
-				 const char *msg, int len, enum date_mode dmode)
+				 const char *msg, int len,
+				 const struct date_mode *dmode)
 {
 	/* currently all placeholders have same length */
 	const int placeholder_len = 2;
@@ -711,16 +712,16 @@ static size_t format_person_part(struct strbuf *sb, char part,
 		strbuf_addstr(sb, show_ident_date(&s, dmode));
 		return placeholder_len;
 	case 'D':	/* date, RFC2822 style */
-		strbuf_addstr(sb, show_ident_date(&s, DATE_RFC2822));
+		strbuf_addstr(sb, show_ident_date(&s, DATE_MODE(RFC2822)));
 		return placeholder_len;
 	case 'r':	/* date, relative */
-		strbuf_addstr(sb, show_ident_date(&s, DATE_RELATIVE));
+		strbuf_addstr(sb, show_ident_date(&s, DATE_MODE(RELATIVE)));
 		return placeholder_len;
 	case 'i':	/* date, ISO 8601-like */
-		strbuf_addstr(sb, show_ident_date(&s, DATE_ISO8601));
+		strbuf_addstr(sb, show_ident_date(&s, DATE_MODE(ISO8601)));
 		return placeholder_len;
 	case 'I':	/* date, ISO 8601 strict */
-		strbuf_addstr(sb, show_ident_date(&s, DATE_ISO8601_STRICT));
+		strbuf_addstr(sb, show_ident_date(&s, DATE_MODE(ISO8601_STRICT)));
 		return placeholder_len;
 	}
 
@@ -933,7 +934,7 @@ static void rewrap_message_tail(struct strbuf *sb,
 static int format_reflog_person(struct strbuf *sb,
 				char part,
 				struct reflog_walk_info *log,
-				enum date_mode dmode)
+				const struct date_mode *dmode)
 {
 	const char *ident;
 
@@ -1185,7 +1186,7 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 			if (c->pretty_ctx->reflog_info)
 				get_reflog_selector(sb,
 						    c->pretty_ctx->reflog_info,
-						    c->pretty_ctx->date_mode,
+						    &c->pretty_ctx->date_mode,
 						    c->pretty_ctx->date_mode_explicit,
 						    (placeholder[1] == 'd'));
 			return 2;
@@ -1200,7 +1201,7 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 			return format_reflog_person(sb,
 						    placeholder[1],
 						    c->pretty_ctx->reflog_info,
-						    c->pretty_ctx->date_mode);
+						    &c->pretty_ctx->date_mode);
 		}
 		return 0;	/* unknown %g placeholder */
 	case 'N':
@@ -1251,11 +1252,11 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 	case 'a':	/* author ... */
 		return format_person_part(sb, placeholder[1],
 				   msg + c->author.off, c->author.len,
-				   c->pretty_ctx->date_mode);
+				   &c->pretty_ctx->date_mode);
 	case 'c':	/* committer ... */
 		return format_person_part(sb, placeholder[1],
 				   msg + c->committer.off, c->committer.len,
-				   c->pretty_ctx->date_mode);
+				   &c->pretty_ctx->date_mode);
 	case 'e':	/* encoding */
 		if (c->commit_encoding)
 			strbuf_addstr(sb, c->commit_encoding);