git subtree push mangles commit message newlines

Hi all,

I recently started using git-subtree and ran into a small issue when
pushing to the subtree repository: newlines in the commit message seem
not to be preserved.

I was using git-subtree from [1], but can also reproduce this with the
version from the main git source-tree commit 634392b26275fe, and I
don't see any changes that could affect this behaviour since then.

Best,
Jaap

[1] https://github.com/helmo/git/tree/subtree-updates/contrib/subtree