git-codereview: make tests pass on windows

- "git gofmt" uses "git checkout-index" command that is broken if
  used with many arguments - limit number of arguments passed to
  "git checkout-index";
- change / into \ on windows when matching git output on windows;
- add .gitattributes to test repository to make it look more like
  all real Go repositories;
- make sure no files left behind in /tmp on windows.

Fixes golang/go#9433.

Change-Id: Ifceacfffd1cd4b02190d7e941611d0d0f3c48cdd
Reviewed-on: https://go-review.googlesource.com/2089
Reviewed-by: Russ Cox <rsc@golang.org>
4 files changed
tree: c9d2cf188fad477854baeb37af65b85195f9b9c1
  1. .gitattributes
  2. README
  3. git-codereview/