git clone was failing with 'invalid object name HEAD' if ran from cmd.exe directly

environment.c caches results of many getenv calls.
Under MinGW setenv(X) invalidates all previous values returned by getenv(X)
so cached values become dangling pointers.

Replaced all setenv(GIT_DIR, ...) with set_git_dir

Signed-off-by: Dmitry Kakurin <Dmitry.Kakurin@gmail.com>

setup.c

@@ -332,7 +332,7 @@ const char *setup_git_directory_gently(int *nongit_ok)
 			inside_git_dir = 1;
 			if (!work_tree_env)
 				inside_work_tree = 0;
-			setenv(GIT_DIR_ENVIRONMENT, ".", 1);
+			set_git_dir(".");
 			return NULL;
 		}
 		chdir("..");