t4205: correctly test %(describe:abbrev=...)

The pretty format %(describe:abbrev=<number>) tells describe to use
at least <number> digits of the oid to generate the human-readable
format of the commit-ish.

There are three things to test here:
  - Check that we can describe a commit that is not tagged (that is,
    for example our HEAD is at least one commit ahead of some reachable
    commit which is tagged) with at least <number> digits of the oid
    being used for describing it.

  - Check that when using such a commit-ish, we always use at least
    <number> digits of the oid to describe it.

  - Check that we can describe a tag. This just gives the name of the
    tag irrespective of abbrev (abbrev doesn't make sense here).

Do this, instead of the current test which only tests the last case.

Helped-by: Junio C Hamano <gitster@pobox.com>
Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Hariom Verma <hariom18599@gmail.com>
Signed-off-by: Kousik Sanagavarapu <five231003@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4205-log-pretty-formats.sh

@@ -1012,10 +1012,25 @@ test_expect_success '%(describe:tags) vs git describe --tags' '
 
 test_expect_success '%(describe:abbrev=...) vs git describe --abbrev=...' '
 	test_when_finished "git tag -d tagname" &&
+
+	# Case 1: We have commits between HEAD and the most recent tag
+	#	  reachable from it
+	test_commit --no-tag file &&
+	git describe --abbrev=15 >expect &&
+	git log -1 --format="%(describe:abbrev=15)" >actual &&
+	test_cmp expect actual &&
+
+	# Make sure the hash used is at least 15 digits long
+	sed -e "s/^.*-g\([0-9a-f]*\)$/\1/" <actual >hexpart &&
+	test 16 -le $(wc -c <hexpart) &&
+
+	# Case 2: We have a tag at HEAD, describe directly gives the
+	#	  name of the tag
 	git tag -a -m tagged tagname &&
 	git describe --abbrev=15 >expect &&
 	git log -1 --format="%(describe:abbrev=15)" >actual &&
-	test_cmp expect actual
+	test_cmp expect actual &&
+	test tagname = $(cat actual)
 '
 
 test_expect_success 'log --pretty with space stealing' '