[DOC BUG] git tag uses GIT_COMMITTER_DATE, not GIT_AUTHOR_DATE

> 	header_len = snprintf(header_buf, sizeof(header_buf),
> 			  "object %s\n"
> 			  "type %s\n"
> 			  "tag %s\n"
> 			  "tagger %s\n\n",
> 			  sha1_to_hex(object),
> 			  typename(type),
> 			  tag,
> 			  git_committer_info(IDENT_ERROR_ON_NO_NAME));

> In Git v1.5.4.1, Documentation/git-tag.txt advises:
>
>> To set the date used in future tag objects, set the environment
>> variable GIT_AUTHOR_DATE to one or more of the date and time.  The
>> date and time can be specified in a number of ways; the most common
>> is "YYYY-MM-DD HH:MM".
>
> But this does not work because git tag uses GIT_COMMITTER_DATE instead.
> In builtin-tag.c:
>
>> 	header_len = snprintf(header_buf, sizeof(header_buf),
>> 			  "object %s\n"
>> 			  "type %s\n"
>> 			  "tag %s\n"
>> 			  "tagger %s\n\n",
>> 			  sha1_to_hex(object),
>> 			  typename(type),
>> 			  tag,
>> 			  git_committer_info(IDENT_ERROR_ON_NO_NAME));
>
> i.e. it does not call git_author_info.
>
> The backdating section was added to git-tag.txt in 5040beff,
> and builtin-tag.c already used git_committer_info at that time.
> The older git-tag.sh used tagger=$(git-var GIT_COMMITTER_IDENT).
> Thus it seems the bug is in the documentation, not in the code.