Do you actually want the space? Won't that result in the old contents getting a leading space, or am I misunderstanding the fix? Should we instead check if new_template has a newline and add one if it doesn't?

I think we would want the space since the text is replaced using the replacement_text value. So unless the user consciously adds a space to the end of that value, it will not be included.

Just for reference, using git version 2.25.0, old_contents would be as follows (with the preceding newline):

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
#
# On branch bug/705-extra-newline
# Your branch is up to date with 'origin/bug/705-extra-newline'.
#
# Changes to be committed:
#       modified:   lib/overcommit/hook/prepare_commit_msg/replace_branch.rb
#