[Bug] commit-tree shouldn't append an extra newline to commit messages

Hello,

When doing git commit-tree to manually create a commit object, it can be seen that the resulting commit's message has an extra appended newline (\n) that was not present in the input for either argument -m or -F. This is both undesirable and inconsistent with the git commit porcelain command.

In code this happens in the file "builtin\commit-tree.c" lines #80 and #105 (these lines numbers are the same for master, maint, and next branches). It seems like the calls to "strbuf_complete_line()" should just be removed to preserve the original input message exactly. As far as I can tell removing this call doesn't have any unintended side-effects.

git version 2.13.1.windows.2

Thanks, Ross