git_command: pass GIT_DIR on Windows with /

When using Git under Windows, it seems that Git doesn't always parse
GIT_DIR correctly when it uses the Windows \ form, but does when it
uses / only.

For example, when using worktrees:
$ GIT_DIR='C:\Users\vapier\Desktop\repo\breakpad\tools\test\.git' git worktree list
fatal: not a git repository: ..\..\.repo\worktrees\linux-syscall-support.git\worktrees\test
$ GIT_DIR='C:/Users/vapier/Desktop/repo/breakpad/tools/test/.git' git worktree list
C:/Users/vapier/Desktop/repo/breakpad/.repo/worktrees/linux-syscall-support.git  fd00dbbd0c06 (detached HEAD)
..\..\..\..\..\src\src\third_party\lss\.git                                      fd00dbbd0c06 (detached HEAD)
..\..\..\..\..\tools\test\.git                                                   fd00dbbd0c06 (detached HEAD)

Change-Id: I666c03ae845ecb55d7f9800731ea6987d3e7f401
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/298622


Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>