add git hooks for submodules

A much less intrusive version of #3274 that has effect on checkout / pull / merge,
rather than setup.py.  It does require a user install step,
because you cannot add active hooks to a repo.

git-hooks/README.md

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+git hooks for IPython
+
+add these to your `.git/hooks`
+
+For now, we just have `post-checkout` and `post-merge`,
+both of which just update submodules,
+so make sure that you have a fully synced repo whenever you checkout or pull.
+
+To use these hooks, you can symlink or copy them to your `.git/hooks` directory.
+
+    ln -s ../../git-hooks/post-checkout .git/hooks/post-checkout
+    ln -s ../../git-hooks/post-merge .git/hooks/post-merge
+

git-hooks/post-checkout

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+#!/bin/sh
+
+git submodule init
+git submodule update

git-hooks/post-merge

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+#!/bin/sh
+
+git submodule init
+git submodule update