Allow environment variables to be unset in the processes started by run_command

Allow environment variables to be unset in the processes started by run_command

To unset a variable, just specify its name, without "=". For example:

    const char *env[] = {"GIT_DIR=.git", "PWD", NULL};
    const char *argv[] = {"git-ls-files", "-s", NULL};
    int err = run_command_v_opt_cd_env(argv, RUN_GIT_CMD, ".", env);

The PWD will be unset before executing git-ls-files.

Signed-off-by: Alex Riesen <redacted>
Signed-off-by: Junio C Hamano <redacted>