py3: spawn all python instances with legacy stdio enabled on Windows This fixes 6 more tests. See ef7119cd4965.

setup: avoid attempting to invoke the system-wide hg.exe on Windows On Windows, the executable in the current directory gets priority over anything in $PATH (both for cmd.exe and MSYS). That means, the former code was launching the local hg.exe instead of the system-wide one, if it was previously built. If that failed, it then fell back to the local hg code, but run through python.exe. I'm not sure what it is about ef7119cd4965, but that started throwing up a messagebox that python37.dll couldn't be loaded. (And indeed, python37 is not in $PATH by default.) Invoking the local hg via the current python avoids that.

delta: ignore base whose chains already don't match expectations If we know the existing chain does not match our criteria, there is no point to build a delta to append. This is especially useful when dealing with a full text much smaller than its parent. In that case, the parent chain is probably already too large. example affected manifest write before: 1.421005s after: 0.815520s (-42%)