Makefile: allow "bin-wrappers/" directory to exist

The "bin-wrappers/" directory gets created by our build system and is
populated with one script for each of our binaries. There isn't anything
inherently wrong with the current layout, but it is somewhat hard to
adapt for out-of-tree build systems.

Adapt the layout such that our "bin-wrappers/" directory always exists
and contains our "wrap-for-bin.sh" script to make things a little bit
easier for subsequent steps.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/CodingGuidelines

@@ -583,7 +583,7 @@ For C programs:
    Run `GIT_DEBUGGER=1 ./bin-wrappers/git foo` to simply use gdb as is, or
    run `GIT_DEBUGGER="<debugger> <debugger-args>" ./bin-wrappers/git foo` to
    use your own debugger and arguments. Example: `GIT_DEBUGGER="ddd --gdb"
-   ./bin-wrappers/git log` (See `wrap-for-bin.sh`.)
+   ./bin-wrappers/git log` (See `bin-wrappers/wrap-for-bin.sh`.)
 
  - The primary data structure that a subsystem 'S' deals with is called
    `struct S`. Functions that operate on `struct S` are named