Upgrade installed Cygwin packages in AppVeyor CI

[dra27]

The Cygwin diffutils packages was updated a few days ago and this is causing a conflict on the AppVeyor CI because the new diff binary depends on an updated Cygwin DLL.

The fix is to add the -g parameter to the call to Cygwin's setup which installs diffutils (and other packages). The effect of this switch is to upgrade all existing packages as well (i.e. sudo apt-get upgrade / sudo yum update). The effect of this is that the Cygwin installation takes noticeably longer, probably owing to having to rebase the entire installation.

Ultimately, this requires the underlying AppVeyor Cygwin installation image to be updated - we can monitor this by looking for early in AppVeyor logs for these lines:

%CYG_ROOT%\bin\bash -lc "cygcheck -dc cygwin"
Cygwin Package Information
Package              Version
cygwin               2.5.2-1
"%CYG_ROOT%\setup-x86.exe" -qgnNdO -R "%CYG_ROOT%" -s "%CYG_MIRROR%" -l "%CYG_CACHE%" -P diffutils -P dos2unix -P gcc-core -P make -P ncurses >NUL %CYG_ROOT%\bin\bash -lc "cygcheck -dc cygwin"
Cygwin Package Information
Package              Version
cygwin               2.6.0-1
call "C:\Program Files\Microsoft SDKs\Windows\v7.1\Bin\SetEnv.cmd" /x64

which allows us to see the version of the Cygwin package before and after update.

[dra27]

Despite the performance hit (though AppVeyor is still returning faster than Travis), I propose merging this quickly to get the Windows CI back up and running...

/cc @avsm, @Chris00. @gasche, @damiendoligez

[avsm]

The change looks good to me, with some notes:

• we'll need a similar changeset in https://github.com/ocaml/ocaml-ci-scripts due to the diffutils change. However the OPAM bugs have been worked around by simply removing diffutils, so perhaps we can live without the extra update time.
• hosting a Cygwin mirror will give us better control over the versioning of packages for build; see Cygwin: host a mirror infrastructure#8

[dra27]

diffutils is only required for running the testsuite so, yes, opam can probably survive!