tests/test-double-merge

Windows: improve performance via buffered I/O

The posixfile_nt code hits the win32 file API directly, which
essentially amounts to performing a system call for every read and
write. This is slow.

We add a C extension that lets us use a Python file object instead,
but preserve our desired POSIX-like semantics (the ability to rename
or delete a file that is being accessed).

If the C extension is not available (e.g. in a VPS environment
without a compiler), we fall back to the posixfile_nt code.

#!/bin/sh

hg init repo
cd repo

echo line 1 > foo
hg ci -qAm 'add foo' -d "1000000 0"

# copy foo to bar and change both files
hg cp foo bar
echo line 2-1 >> foo
echo line 2-2 >> bar
hg ci -m 'cp foo bar; change both' -d "1000000 0"

# in another branch, change foo in a way that doesn't conflict with
# the other changes
hg up -qC 0
echo line 0 > foo
hg cat foo >> foo
hg ci -m 'change foo' -d "1000000 0"

# we get conflicts that shouldn't be there
hg merge --debug

echo "-- foo --"
cat foo

echo "-- bar --"
cat bar