Win32: Thread-safe windows console output

Win32: Thread-safe windows console output

Replace stdout and stderr with a pipe if they point to the console. A
background thread reads from the pipe, handles ANSI escape sequences and
UTF-8 to UTF-16 conversion, then writes to the console.

Global variables are either initialized on startup (single threaded) or
exclusively modified by the background thread. Threads communicate through
the pipe, no further synchronization is necessary.

Due to the byte-oriented pipe, ANSI escape sequences can no longer be
expected to arrive in one piece. Replace the string-based ansi_emulate()
with a simple stateful parser (this also fixes colored diff hunk headers,
which were broken as of commit 2efcc977).

Override isatty to return true for the pipes redirecting to the console.

Exec/spawn obtain the original console handle to pass to the next process
via winansi_get_osfhandle().

All other overrides are gone, the default stdio implementations work as
expected with the piped stdout/stderr descriptors.

Limitations: doesn't track reopened or duped file descriptors, i.e.:
- fdopen(1/2) returns fully buffered streams
- dup(1/2), dup2(1/2) returns normal pipe descriptors (i.e. isatty() =
  false, winansi_get_osfhandle won't return the original console handle)

Currently, only the git-format-patch command uses xfdopen(xdup(1)) (see
"realstdout" in builtin/log.c), but works well with these limitations.

Signed-off-by: Karsten Blees <blees@dcon.de>