annex.fastcopy

annex.fastcopy

Added annex.fastcopy and remote.name.annex-fastcopy config setting. When
set, this allows the copy_file_range syscall to be used, which can eg allow
for server-side copies on NFS. (For fastest copying, also disable
annex.verify or remote.name.annex-verify.)

This is a simple implementation, that does not handle resuming as well as
it possibly could.

It can be used with both local git remotes (including on NFS), and
directory special remotes. Other types of remotes could in theory also
support it, so I've left the config documented as a general thing.