archive: abbreviate substituted commit ids again

Given a file with:

  (define archive-id "$Format:%ct|%h|a$")

and an export-subst attribute, the "%h" results in an full 40-digit
object name instead of the expected 7-digit one.

The export-subst feature requests unabbreviated object names because
that is the low-level default.  The effect was not observable until
v1.7.1.1~17^2~3 (2010-05-03), which taught log --format=%h to respect
the --abbrev option.

Reported-by: Eli Barzilay <eli@barzilay.org>
Tested-by: Eli Barzilay <eli@barzilay.org>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

archive.c

@@ -33,6 +33,7 @@ static void format_subst(const struct commit *commit,
 	struct strbuf fmt = STRBUF_INIT;
 	struct pretty_print_context ctx = {0};
 	ctx.date_mode = DATE_NORMAL;
+	ctx.abbrev = DEFAULT_ABBREV;
 
 	if (src == buf->buf)
 		to_free = strbuf_detach(buf, NULL);

t/t5001-archive-attr.sh

@@ -4,7 +4,7 @@ test_description='git archive attribute tests'
 
 . ./test-lib.sh
 
-SUBSTFORMAT=%H%n
+SUBSTFORMAT='%H (%h)%n'
 
 test_expect_exists() {
 	test_expect_success " $1 exists" "test -e $1"