git-gc: add page

pages/common/git-gc.md

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+# git gc
+
+> Optimise the local repository by cleaning unnecessary files.
+
+- Optimise the repository:
+
+`git gc`
+
+- View full usage:
+
+`git gc --help`
+
+- Aggressively optimise, takes more time:
+
+`git gc --aggressive`
+
+- Do not prune loose objects (prunes by default):
+
+`git gc --no-prune`
+
+- Suppress all output:
+
+`git gc --quiet`