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
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