builtin-tag.c: Fix two memory leaks and minor notation changes.

builtin-tag.c: Fix two memory leaks and minor notation changes.

A repeated call to read_sha1_file was not freing memory
when the buffer was allocated but returned size was zero.

Also, now the program does not allow many -F or -m options,
which was a bug too because it was not freing the memory
allocated for any previous -F or -m options.

Tests are provided for ensuring that only one option
-F or -m is given. Also, another test is shipped here,
to check that "git tag" fails when a non-existing file
is passed to the -F option, something that git-tag.sh
allowed creating the tag with an empty message.

Signed-off-by: Carlos Rica <jasampler@gmail.com>
Acked-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>