Update cheatsheets

tldr/dolt-gc

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# dolt gc
+
+> Search the repository for data that are no longer referenced and no longer needed.
+> More information: <https://docs.dolthub.com/cli-reference/cli#dolt-gc>.
+
+- Clean up unreferenced data from the repository:
+
+`dolt gc`
+
+- Initiate a faster but less thorough garbage collection process:
+
+`dolt gc --shallow`