Cygwin+WindowsVisualDiffApp support

PLATFORM: Cygwin (using Bazaar from Cygwin shell)

WinMerge and other Windows VisualDiff/Merge applications require
Windows/POSIX filenames. Currently, WinMerge is started with
Cygwin-specific pathnames which WinMerge (and other apps) cannot handle.

REQUIRED-CHANGE:
On Cygwin platform, filenames should be converted to DOS/POSIX format.
It would even, if filepath transformation rules could registered for
some platforms or when the specific DiffTool is registered.

PATCH (QUICKFIX):
file:difftool.py
context:DiffTool.run() method
    ...
    # XXX-JE-PATCH: Cygwin-to-DOS Files Names.
    import sys
    if sys.platform == "cygwin":
        # -- CYGWIN-ENIRONMENT: Running system but maybe call Windows apps.
        # REQUIRE: Filename conversion to DOS-like filenames (POSIX).
        # /cygdrive/V/my/FileUnderVersionControl -> V:/my/FileUnderVersionControl
        # /tmp/.../FileUnderVersionControl.1234 -> C:/cygwin/tmp/.../FileUnderVersionControl.1234
        CYGWIN_DOS_PREFIX = "C:/cygwin/" #< BETTER: Auto-detect or in ConfigFile.
        if new_path.startswith("/cygdrive/"):
            new_path2 = new_path.replace("/cygdrive/", "", 1)
            new_path = "%s:%s" % (new_path2[0], new_path2[1:])
        if old_path.startswith("/"):
            old_path = "%s%s" % (CYGWIN_DOS_PREFIX, old_path[1:])
    # XXX-JE-PATCH-END.
    run_tool = [self.command] + diff_opts + [old_path, new_path]
    # XXX-JE-PATCH: Cygwin-to-DOS Files Names.
    debug_visualdiff_call = True
    if debug_visualdiff_call:
        print "Bazaar difftool: %s" % " ".join(run_tool)
    # XXX-JE-PATCH-END.