Running commands in wrong environment

git's start_command (run_command.c) executes a command (e.g. hook) but does not verify that it has properly set up the environment. It seems that in the unlikely case where putenv (run_command.c:117) fails, the command may have undesirable effects e.g. GIT_INDEX_FILE should have been set (interactive pre-commit hooks) but the default index will be used instead. It would be safer not to run the command but just exit in that case.