memory: use core.packedGitWindowSize=32m unless $cfg_git_no_mmap

memory: use core.packedGitWindowSize=32m unless $cfg_git_no_mmap

The default value for core.packedGitWindowSize on a 64-bit platform
where Git was built WITHOUT setting NO_MMAP can lead to unwanted
memory blowout when running many packing threads.

Instead always set the 32-bit value (32 MiB).

However, on some platforms, Git must be built with NO_MMAP in order
to function properly.  Therefore provide an escape hatch in that
if $Girocco::Config::git_no_mmap is set to a true value then always
set core.packedGitWindowSize to 1 MiB instead (which is what happens
when Git is built with NO_MMAP).

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>