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: Jeff King <peff@peff.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
diff --git a/revision.c b/revision.c
index 35eca4a..9fc4e8d 100644
--- a/revision.c
+++ b/revision.c
@@ -1159,8 +1159,10 @@
 		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;
 	}