Honor git commit.template config setting.

Add a hook to `magit-log-edit-mode` to insert the contents of the
`commit.template` file if configured.  This appears to match the
behavior of git from the command line.
As far as I can tell, git does the following:
If the `commit.template` is configured in a `.git/config`, treat a relative
path as relative to the top directory of the repo.
If it is a global config (`~/.gitconfig`) or an absolute path, use that path.

Using this, every time I open magit-edit-log, I get this I'm not sure
about that, but it's hidden in `*Messages*`:

    Buffer `*magit-edit-log*' not visiting a file