$ git-new-workdir /media/kepler-data/home/thomas/tmp/source/binutils/git master master
fatal: Out of memory? mmap failed: No such device
$ echo $?
128
$ showtrans /media/kepler-data
/hurd/nfs kepler.schwinge.homeip.net:/media/data

With sh -x:

[...]
+ ln -s /media/kepler-data/home/thomas/tmp/source/binutils/git/.git/remotes master/.git/remotes
+ ln -s /media/kepler-data/home/thomas/tmp/source/binutils/git/.git/rr-cache master/.git/rr-cache
+ ln -s /media/kepler-data/home/thomas/tmp/source/binutils/git/.git/svn master/.git/svn
+ cd master
+ cp /media/kepler-data/home/thomas/tmp/source/binutils/git/.git/HEAD .git/HEAD
+ git checkout -f master
fatal: Out of memory? mmap failed: No such device

As one can easily guess (and confirm with rpctrace), git tries to mmap a file via the nfs translator, this fails, and it isn't prepared to cope with that:

[...]
  88->dir_lookup (".git/objects/pack/pack-37ca560e7877fa0cc6e5ddcd556aa73e5a3e3f40.idx" 2049 0) = 0 3 "/media/kepler-data/home/thomas/tmp/source/binutils/git/.git/objects/pack/pack-37"  (null)
  62->dir_lookup ("media/kepler-data/home/thomas/tmp/source/binutils/git/.git/objects/pack/pack-37c" 2049 0) = 0 1 "/home/thomas/tmp/source/binutils/git/.git/objects/pack/pack-37ca560e7877fa0cc6e5"   61
  61->dir_lookup ("home/thomas/tmp/source/binutils/git/.git/objects/pack/pack-37ca560e7877fa0cc6e5d" 2049 0) = 0 1 ""   84
task3741-> 3206 (pn{ 33}) = 0
  84->term_getctty () = 0xfffffed1 ((ipc/mig) bad request message ID)
  84->io_stat_request () = 0 {1 704 0 36308992 0 0 -1 33060 1 1000 1000 4712 0 1307711395 0 1307657003 0 1307657003 0 4096 16 0 1000 0 0 100663296 1836017780 29537 0 0 0 0}
  84->io_map_request () = 0x4000002d (Operation not supported)
  84->io_map_request () = 0x4000002d (Operation not supported)
  76->io_write_request ("fatal: Out of memory? mmap failed: No such device
" -1) = 0 50
  64->proc_mark_exit_request (32768 0) = 0
task3741-> 2008 () = 0
Child 3741 exited with 128

This is the libnetfs: io map issue.

There is a NO_MMAP conditional in Git's source code, but it is a compile-time conditional. The fallback code in compat/mmap.c:git_mmap only supports MAP_PRIVATE, and simply preads in the requested portion of a file. This could be made a runtime fallback, too.