Update cheatsheets

arc

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# arc
+
+> Arcanist: a CLI for Phabricator.
+> More information: <https://secure.phabricator.com/book/phabricator/article/arcanist/>.
+
+- Send the changes to Differential for review:
+
+`arc diff`
+
+- Show pending revision information:
+
+`arc list`
+
+- Update Git commit messages after review:
+
+`arc amend`
+
+- Push Git changes:
+
+`arc land`