dirstate: make dirstate flags char be unsigned Since https://phab.mercurial-scm.org/D11387, `CC='clang -Werror' make local` has started failing like this: ``` mercurial/cext/util.h:41:50: error: implicit conversion from 'int' to 'char' changes value from 128 to -128 [-Werror,-Wconstant-conversion] static const char dirstate_flag_rust_special = 1 << 7; ~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~^~~~ ``` This patch fixes that by making the flags be an unsigned char. That also matches the `bool` typedef we have in `util.h`, which seems good since many of the `dirstate_item_c_*()` functions return a `bool`. Differential Revision: https://phab.mercurial-scm.org/D11444