git-contrib: add page (#6064)

pages/common/git-contrib.md

@@ -0,0 +1,9 @@
+# git contrib
+
+> Display commits from a specific author.
+> Part of `git-extras`.
+> More information: <https://github.com/tj/git-extras/blob/master/Commands.md#git-contrib>.
+
+- Display all commit hashes and their corresponding commit messages from a specific author:
+
+`git contrib {{author}}`