Make verify-tag a builtin.

Make verify-tag a builtin.

This replaces "git-verify-tag.sh" with "builtin-verify-tag.c".

Testing relies on the "git tag -v" tests calling this command.

A temporary file is needed when calling to gpg, because git is
already creating detached signatures (gpg option -b) to sign tags
(instead of leaving gpg to add the signature to the file by itself),
and those signatures need to be supplied in a separate file to be
verified by gpg.

The program uses git_mkstemp to create that temporary file needed by
gpg, instead of the previously used "$GIT_DIR/.tmp-vtag", in order to
allow the command to be used in read-only repositories, and also
prevent other instances of git to read or remove the same file.

Signal SIGPIPE is ignored because the program sometimes was
terminated because that signal when writing the input for gpg.

The command now can receive many tag names to be verified.
Documentation is also updated here to reflect this new behaviour.

Signed-off-by: Carlos Rica <jasampler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>