git-commit: fix to lack of newline separation

There was an issue where the trailers were not properly separated from
the commit message itself. In fact, a new line is not added if there
are not trailers either.

Add a fix to add a newline after the commit message. Also adding test
cases to test 0, 1, or 2 trailers in a git commit.

Signed-off-by: Fintan Halpenny <fintan.halpenny@gmail.com>
X-Clacks-Overhead: GNU Terry Pratchett

git-commit/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "git-commit"
-version = "0.2.0"
+version = "0.3.0"
 license = "MIT OR Apache-2.0"
 edition = "2021"
 authors = [