git format patch: add page (#2351)

pages/common/git-format-patch.md

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+# git format-patch
+
+> Prepare .patch files. Useful when emailing commits elsewhere.
+> See also `git am`, which can apply generated .patch files.
+
+- Create an auto-named .patch file for all the unpushed commits:
+
+`git format-patch {{origin}}`
+
+- Write a .patch file for all the commits between 2 revisions to stdout:
+
+`git format-patch {{revision_1}}..{{revision_2}}`
+
+- Write a .patch file for the 3 latest commits:
+
+`git format-patch -{{3}}`