Make packfile_unpack_header more generic
On the way, store the fd and the size in the mwindow file.
Signed-off-by: Carlos Martín Nieto <carlos@cmartin.tk>
Factor out the mmap window code
This code is useful for more things than just the packfile handling
code. Factor it out so it can be reused.
Signed-off-by: Carlos Martín Nieto <carlos@cmartin.tk>
