Fix: fail-gradefully if there are not valid commits

tests/cram/test_fail_gracefully_if_no_commits.t

@@ -0,0 +1,53 @@
+We need to run semv in a git repository.
+  $ bash "$TESTDIR"/setup.sh
+  Initialized empty Git repository in */.git/ (glob)
+  [master (root-commit) *] docs(readme): Add readme (glob)
+   1 file changed, 1 insertion(+)
+   create mode 100644 README.md
+
+We now have a (non working) initial version v0.0.0 (note however, that this is
+not standard and you might rather want to pick v1.0.0).
+  $ semv
+  WARNING: No changes for new version
+  [1]
+We will now add a few commits and then get a new version.
+  $ echo "print('Hello')" > myscript.py
+  $ git add myscript.py
+  $ git commit -m 'feat(myscript): Print a message'
+  [master *] feat(myscript): Print a message (glob)
+   1 file changed, 1 insertion(+)
+   create mode 100644 myscript.py
+  $ echo "print('Hello world!')" > myscript.py
+  $ git add myscript.py
+  $ git commit -m 'fix(myscript): Better message printed'
+  [master *] fix(myscript): Better message printed (glob)
+   1 file changed, 1 insertion(+), 1 deletion(-)
+
+We now have two commits: The first implements a new feature, the second one
+fixes the initial implementation. Let's assume we want to perform a release
+now. We can print the new version (without writing anything yet) by running semv:
+  $ semv
+  v0.1.0 (no-eol)
+As you can see, the output has no final newline character. That is because the
+intended usage of semv would be to cause a new version tag like so:
+  $ git tag $(semv)
+This illustrates a core feature of semv: It will not perform any modifications
+to your system.
+
+  $ semv
+  WARNING: No changes for new version
+  [1]
+
+Now let's add a commit that has an invalid message
+  $ echo "This tool just greets the world" >> README.md
+  $ git add README.md
+  $ git commit -m 'readme: Add some more info about our tool'
+  [master *] readme: Add some more info about our tool (glob)
+   1 file changed, 1 insertion(+)
+  $ git tag
+  v0.0.0
+  v0.1.0
+  $ semv
+  WARNING: Invalid commit: * readme: Add some more info about our tool (glob)
+  WARNING: No changes for new version
+  [1]