git-merge-base: add page (#9382)

Co-authored-by: Axel Navarro <navarroaxel@gmail.com>

pages/common/git-merge-base.md

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+# git merge-base
+
+> Find a common ancestor of two commits.
+> More information: <https://git-scm.com/docs/git-merge-base>.
+
+- Print the best common ancestor of two commits:
+
+`git merge-base {{commit_1}} {{commit_2}}`
+
+- Output all best common ancestors of two commits:
+
+`git merge-base --all {{commit_1}} {{commit_2}}`
+
+- Check if a commit is an ancestor of a specific commit:
+
+`git merge-base --is-ancestor {{ancestor_commit}} {{commit}}`