tag: generate useful reflog message

When tags are created with `--create-reflog` or with the option
`core.logAllRefUpdates` set to 'always', a reflog is created for them.
So far, the description of reflog entries for tags was empty, making the
reflog hard to understand. For example:
6e3a7b3 refs/tags/test@{0}:

Now, a reflog message is generated when creating a tag, following the
pattern "tag: tagging <short-sha1> (<description>)". If
GIT_REFLOG_ACTION is set, the message becomes "$GIT_REFLOG_ACTION
(<description>)" instead. If the tag references a commit object, the
description is set to the subject line of the commit, followed by its
commit date. For example:
6e3a7b3 refs/tags/test@{0}: tag: tagging 6e3a7b3398 (Git 2.12-rc0, 2017-02-03)

If the tag points to a tree/blob/tag objects, the following static
strings are taken as description:

 - "tree object"
 - "blob object"
 - "other tag object"

Signed-off-by: Cornelius Weig <cornelius.weig@tngtech.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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