git-gc: add page (#1741)

pages/common/git-gc.md

@@ -0,0 +1,23 @@
+# git gc
+
+> Optimise the local repository by cleaning unnecessary files.
+
+- Optimise the repository:
+
+`git gc`
+
+- 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`
+
+- View full usage:
+
+`git gc --help`