log: respect date_mode_explicit with --format:%gd

When we show a reflog selector (e.g., via "git log -g"), we
perform some DWIM magic: while we normally show the entry's
index (e.g., HEAD@{1}), if the user has given us a date
with "--date", then we show a date-based select (e.g.,
HEAD@{yesterday}).

However, we don't want to trigger this magic if the
alternate date format we got was from the "log.date"
configuration; that is not sufficiently strong context for
us to invoke this particular magic. To fix this, commit
f4ea32f (improve reflog date/number heuristic, 2009-09-24)
introduced a "date_mode_explicit" flag in rev_info. This
flag is set only when we see a "--date" option on the
command line, and we a vanilla date to the reflog code if
the date was not explicit.

Later, commit 8f8f547 (Introduce new pretty formats %g[sdD]
for reflog information, 2009-10-19) added another way to
show selectors, and it did not respect the date_mode_explicit
flag from f4ea32f.

This patch propagates the date_mode_explicit flag to the
pretty-print code, which can then use it to pass the
appropriate date field to the reflog code. This brings the
behavior of "%gd" in line with the other formats, and means
that its output is independent of any user configuration.

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

commit.h

@@ -82,6 +82,7 @@ struct pretty_print_context {
 	const char *after_subject;
 	int preserve_subject;
 	enum date_mode date_mode;
+	unsigned date_mode_explicit:1;
 	int need_8bit_cte;
 	int show_notes;
 	struct reflog_walk_info *reflog_info;