Adds documentation for git rebase and git fetch

pages/common/git-fetch.md

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+# git fetch
+
+> Download objects and refs from a remote repository
+
+- Fetch the latest changes from all remote git servers
+
+`git fetch --all`

pages/common/git-rebase.md

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+# git rebase
+
+> Apply local commits on top of another branch's history
+
+- Rebase your local branch with the latest changes in master
+
+`git rebase -i master` (Then follow the directions in the terminal for merge conflicts)