git-merge-*: add page (#9386)

* git merge-into: add page

* git merge-repo: add page

* Apply suggestions for git-merge-into

* Apply suggestions for git-merge-repo

pages/common/git-merge-into.md

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+# git merge-into
+
+> Merge one branch into another branch.
+> Part of `git-extras`.
+> More information: <https://github.com/tj/git-extras/blob/master/Commands.md#git-merge-into>.
+
+- Merge a source branch into a specific destination branch:
+
+`git merge-into {{source_branch}} {{destination_branch}}`
+
+- Merge current branch into a specific destination branch:
+
+`git merge-into {{destination_branch}}`

pages/common/git-merge-repo.md

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+# git merge-repo
+
+> Merge two repository histories.
+> Part of `git-extras`.
+> More information: <https://github.com/tj/git-extras/blob/master/Commands.md#git-merge-repo>.
+
+- Merge a repository's branch into the current repository's directory:
+
+`git merge-repo {{path/to/repo}} {{branch_name}} {{path/to/directory}}`
+
+- Merge a remote repository's branch into the current repository's directory, not preserving history:
+
+`git merge-repo {{path/to/remote_repo}} {{branch_name}} .`