Create a memory-backed disk in OSX. Useful for sending git filter-branch
to it to speed things up.
# to mount
hdiutil attach -nomount ram://$(( 128 * 1024 * 2 )) # returns device with 128 MB (in 512k blocks)
diskutil erasevolume JHFS+ "tmp" /dev/disk1 # whatever the hdiutil command returns
# when done
diskutil unmount /dev/disk1
hdiutil detach /dev/disk1