pretty: provide short date format

Add the placeholders %as and %cs to format author date and committer
date, respectively, without the time part, like --date=short does, i.e.
like YYYY-MM-DD.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pretty-formats.txt

@@ -169,6 +169,7 @@ The placeholders are:
 '%at':: author date, UNIX timestamp
 '%ai':: author date, ISO 8601-like format
 '%aI':: author date, strict ISO 8601 format
+'%as':: author date, short format (`YYYY-MM-DD`)
 '%cn':: committer name
 '%cN':: committer name (respecting .mailmap, see
 	linkgit:git-shortlog[1] or linkgit:git-blame[1])
@@ -181,6 +182,7 @@ The placeholders are:
 '%ct':: committer date, UNIX timestamp
 '%ci':: committer date, ISO 8601-like format
 '%cI':: committer date, strict ISO 8601 format
+'%cs':: committer date, short format (`YYYY-MM-DD`)
 '%d':: ref names, like the --decorate option of linkgit:git-log[1]
 '%D':: ref names without the " (", ")" wrapping.
 '%S':: ref name given on the command line by which the commit was reached

pretty.c

@@ -731,6 +731,9 @@ static size_t format_person_part(struct strbuf *sb, char part,
 	case 'I':	/* date, ISO 8601 strict */
 		strbuf_addstr(sb, show_ident_date(&s, DATE_MODE(ISO8601_STRICT)));
 		return placeholder_len;
+	case 's':
+		strbuf_addstr(sb, show_ident_date(&s, DATE_MODE(SHORT)));
+		return placeholder_len;
 	}
 
 skip:

t/t4205-log-pretty-formats.sh

@@ -533,6 +533,12 @@ test_expect_success 'ISO and ISO-strict date formats display the same values' '
 	test_cmp expected actual
 '
 
+test_expect_success 'short date' '
+	git log --format=%ad%n%cd --date=short >expected &&
+	git log --format=%as%n%cs >actual &&
+	test_cmp expected actual
+'
+
 # get new digests (with no abbreviations)
 test_expect_success 'set up log decoration tests' '
 	head1=$(git rev-parse --verify HEAD~0) &&