tests: use 'hghave execbit' for tests that manipulate x bit in file system

tests: use 'hghave symlink' for tests using symlinks

windows: use umask 022 in debugstate output debugstate would always report files as mode 666 or 777 on Windows. umask is not used on Windows, but faking and using a defalt value of 022 matches what the test suite uses on Unix.

dispatch: exit with 8-bit exit code The exit code returned from a program to the shell is unsigned 8-bit, but Mercurial would sometimes try to exit with negative numbers or None. sys.exit on Unix will convert that to 8-bit exit codes, but on Windows negative values showed up as 0. The exit code is now explicitly converted to unsigned 8-bit.

tests: don't let time go back before 0 o'clock in bisect3.t Windows agree that it was Thu Jan 01 00:00:00 1970 +0000 but it also claims that it was Thu Dec 31 23:59:59 1969 -0000

diff: always use / in paths in diff Subrepo diffs would sometimes use backslash on windows.

cmdutil: don't use repr on paths in pathauditor - it looks strange on windows