Trac #14031: Provide proper Cygwin rebasing scripts
Trac #14495: Improvement to --optional doctests
Trac #14465: Cleanup source.rst and include instructions for Cygwin
Trac #14465: Install Sage from source: documentation review patch
Fix bash syntax in sage -upgrade
5.9.rc1
5.9.rc1
Merge branches 'sage-5.9.rc1' and 'sage_scripts-5.9.rc1' into master
5.9.rc1: update git specific files

These currently include
 * VERSION.txt
 * package-version.txt
 * checksums.ini