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

dirstate: introduce a `set_clean` method on dirstate's map and items This method is the "reverse" of "set possibly dirty", and can be used to more accurately other call that the dirstate was making. It is currently heavily influenced by its origin. Differential Revision: https://phab.mercurial-scm.org/D11421

dirstate: extract the logic to check file/dirname collision when adding a file Differential Revision: https://phab.mercurial-scm.org/D11420