Broken link in 'Git Commands' appendix

[zigarn]

Git-Commands-Basic-Snapshotting#git-difftool: broken link to '_git_difftool'
Should be a link to:

• Customizing-Git-Git-Configuration#External-Merge-and-Diff-Tools
• or 'Git Diff in an External Tool' note in Git-Basics-Recording-Changes-to-the-Repository#Viewing-Your-Staged-and-Unstaged-Changes

[YueLinHo]

The same issue with #454.
Let's close this issue and go there?