git: fix build with with no threads

When building git with toolchain that don't have thread support, we
get following build errors:

      CC builtin/hash-object.o
  builtin/grep.c: In function 'grep_submodule_launch':
  builtin/grep.c:596:34: error: dereferencing pointer to incomplete type 'struct work_item'
    status = capture_command(&cp, &w->out, 0);
                                    ^
  builtin/grep.c: In function 'grep_submodule':
  builtin/grep.c:644:20: error: storage size of 'w' isn't known
     struct work_item w;
                      ^
  make[2]: *** [builtin/grep.o] Error 1
  make[2]: *** Waiting for unfinished jobs....

Add two upstream patches to fix this issue.

Fixes:
  http://autobuild.buildroot.net/results/94b/94bce9a99a5ce9894a6918774ab75e23d12c1394/



Signed-off-by: Rahul Bedarkar <rahul.bedarkar@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>