Allocate enough memory for the terminator in commit parsing

Allocate enough memory for the terminator in commit parsing

Also allow space for the null-terminator when allocating the buffer in
packfile_unpack_compressed. Up to now, the last newline had served as
a terminator, but 858ef372 searches for a double-newline and exposes
the problem.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>