Update comments in gitattributes and gitignore_global
Rename gitattributes to gitattributes_global for consistency
Update header comment for gitconfig
Add core.attributesfile referencing ~/.gitattributes_global
Move to color.ui instead of individual color.* settings
Move difftool and mergetool selection to platform-specific section
merge.summary is deprecated; add merge.log and merge.keepBackup
Clean up comments
Add WinMerge difftool/mergetool configuration
Move safecrlf, excludesfile, and attributesfile to generic section