Merge branch 'cygwin'

Merge branch 'cygwin'

* cygwin:
  Assorted cleanups to Luke's cygwin fixes.
  Makefile: work with cygwin on different windows versions.
  .gitignore sanity.
  Makefile:  On Windows, executable files must end with .exe.
  client.py:  Windows files don't support ':', so rename cachedir.
  index.py:  os.rename() fails on Windows if dstfile already exists.
  Don't try to rename tmpfiles into existing open files.
  helpers.py:  Cygwin doesn't support `hostname -f`, use `hostname`.
  cmd-index.py:  Retry os.open without O_LARGEFILE if not supported.
  Makefile:  Build on Windows under Cygwin.