ref-filter: add new "describe" atom

Duplicate the logic of %(describe) and friends from pretty to
ref-filter. In the future, this change helps in unifying both the
formats as ref-filter will be able to do everything that pretty is doing
and we can have a single interface.

The new atom "describe" and its friends are equivalent to the existing
pretty formats with the same name.

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>

Documentation/git-for-each-ref.txt

@@ -258,6 +258,29 @@ ahead-behind:<committish>::
 	commits ahead and behind, respectively, when comparing the output
 	ref to the `<committish>` specified in the format.
 
+describe[:options]::
+	A human-readable name, like linkgit:git-describe[1];
+	empty string for undescribable commits. The `describe` string may
+	be followed by a colon and one or more comma-separated options.
++
+--
+tags=<bool-value>;;
+	Instead of only considering annotated tags, consider
+	lightweight tags as well; see the corresponding option in
+	linkgit:git-describe[1] for details.
+abbrev=<number>;;
+	Use at least <number> hexadecimal digits; see the corresponding
+	option in linkgit:git-describe[1] for details.
+match=<pattern>;;
+	Only consider tags matching the given `glob(7)` pattern,
+	excluding the "refs/tags/" prefix; see the corresponding option
+	in linkgit:git-describe[1] for details.
+exclude=<pattern>;;
+	Do not consider tags matching the given `glob(7)` pattern,
+	excluding the "refs/tags/" prefix; see the corresponding option
+	in linkgit:git-describe[1] for details.
+--
+
 In addition to the above, for commit and tag objects, the header
 field names (`tree`, `parent`, `object`, `type`, and `tag`) can
 be used to specify the value in the header field.