improve reflog date/number heuristic

When we show a reflog, we have two ways of naming the entry:
by sequence number (e.g., HEAD@{0}) or by date (e.g.,
HEAD@{10 minutes ago}). There is no explicit option to set
one or the other, but we guess based on whether or not the
user has provided us with a date format, showing them the
date version if they have done so, and the sequence number
otherwise.

This usually made sense if the use did something like "git
log -g --date=relative". However, it didn't make much sense
if the user set the date format using the log.date config
variable; in that case, all of their reflogs would end up as
dates.

This patch records the source of the date format and only
triggers the date-based view if --date= was given on the
command line.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

log-tree.c

@@ -390,7 +390,9 @@ void show_log(struct rev_info *opt)
 			 */
 			show_reflog_message(opt->reflog_info,
 				    opt->commit_format == CMIT_FMT_ONELINE,
-				    opt->date_mode);
+				    opt->date_mode_explicit ?
+					opt->date_mode :
+					DATE_NORMAL);
 			if (opt->commit_format == CMIT_FMT_ONELINE)
 				return;
 		}

revision.c

@@ -1159,8 +1159,10 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 		revs->simplify_history = 0;
 	} else if (!strcmp(arg, "--relative-date")) {
 		revs->date_mode = DATE_RELATIVE;
+		revs->date_mode_explicit = 1;
 	} else if (!strncmp(arg, "--date=", 7)) {
 		revs->date_mode = parse_date_format(arg + 7);
+		revs->date_mode_explicit = 1;
 	} else if (!strcmp(arg, "--log-size")) {
 		revs->show_log_size = 1;
 	}

revision.h

@@ -81,7 +81,8 @@ struct rev_info {
 			show_merge:1,
 			abbrev_commit:1,
 			use_terminator:1,
-			missing_newline:1;
+			missing_newline:1,
+			date_mode_explicit:1;
 	enum date_mode date_mode;
 
 	unsigned int	abbrev;