Do not undo tags when git state is dirty

When using `tito tag --undo` in a repository with a dirty
`git` state, no action should be taken.

Signed-off-by: Steve Kuznetsov <skuznets@redhat.com>

src/tito/common.py

@@ -544,12 +544,30 @@ def undo_tag(tag):
     commit. Assumes you have taken necessary precautions to ensure this is
     what you want to do.
     """
+    if not is_git_state_clean():
+        error_out("Cannot undo a tag when the current git state is not clean!")
+
     # Using --merge here as it appears to undo the changes in the commit,
     # but preserve any modified files:
     output = run_command("git tag -d %s && git reset --merge HEAD^1" % tag)
     print(output)
 
 
+def is_git_state_clean():
+    """
+    Determines if the state of the current git repository is clean or not.
+    """
+    (status, _) = getstatusoutput("git diff-index --quiet HEAD")
+    if status != 0:
+        return False
+
+    (status, output) = getstatusoutput("git ls-files --exclude-standard --others")
+    if len(output) > 0 or status > 0:
+        return False
+
+    return True
+
+
 def get_remote_tag_sha1(tag):
     """
     Get the SHA1 referenced by this git tag in the remote git repo.