Add missing inside_work_tree setting in setup_git_directory_gently

When both GIT_DIR and GIT_WORK_TREE are set, and
setup_git_directory_gently() changes the current working
directory accordingly, it should also set inside_work_tree = 1.

Without this, work_tree handling code in setup_git_directory()
will be activated. If you stay in root work tree (no prefix),
it does not harm. It does if you work from a subdirectory though.
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

setup.c

@@ -240,6 +240,7 @@ const char *setup_git_directory_gently(int *nongit_ok)
 			if (chdir(work_tree_env) < 0)
 				die ("Could not chdir to %s", work_tree_env);
 			strcat(buffer, "/");
+			inside_work_tree = 1;
 			return retval;
 		}
 		if (nongit_ok) {