`git commit -v`, which I use by default, adds a diff to the bottom of
the commit message, like so:

    # ------------------------ >8 ------------------------
    # Do not modify or remove the line above.
    # Everything below it will be ignored.
    diff --git a/file.go b/file.go
    -a bunch
    -of diff'd
    +code

prepare-commit-msg was not properly handling this scenario. Adjust it to
inject its recommendations into the middle of the commit message instead
of always appending them to the end. Also make it resilient to changes
in the core.commentChar Git configuration option.

Release note: None