Add some more items to the FAQ

[rmjarvis]

I'm adding some additional items to the FAQ based on the breakout session I gave today at the LSST DESC sprint week.

Specifically:

• Add an item about git add -p
• Add an item about using git commit --amend and git rebase -i to fix commit messages.
• Add an item about using git commit --amend and git rebase -i to add content to commits.
• Add an item about some things people might want to put in their .gitconfig file.
• Mention git pull -r in the existing item about what to do when git push is blocked.
• Fix a link error in one of the TOC items.