git-delete-squashed-branches: add page (#10988)

pages/common/git-delete-squashed-branches.md

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+# git delete-squashed-branches
+
+> Delete branches that have been "squashed-merged" into a specified branch and checkout. If no branch is specified, default to the currently checked out branch.
+> Part of `git-extras`.
+> More information: <https://github.com/tj/git-extras/blob/master/Commands.md#git-delete-squashed-branches>.
+
+- Delete all branches that were "squash-merged" into the current checked out branch:
+
+`git delete-squashed-branches`
+
+- Delete all branches that were "squash-merged" into a specific branch:
+
+`git delete-squashed-branches {{branch_name}}`