Fix generating dlt version tag

Update git describe to always produce some output. This allows to
ignore git warnings and errors.

Fix problem when git warning about tag get placed in C++ header and
a syntax error breaks build.

Example git output that prodcues compilation errors:
> git describe --tags
warning: tag 'haleytek/v2.18.5' is externally known as 'v2.18.5'
v2.18.5-19-g344c37b

util/create_dlt_version_h.py

@@ -20,19 +20,13 @@
 
 def get_cmd(cmd, cwd):
     return subprocess.check_output(cmd, cwd=cwd, shell=True,
-                                   stderr=subprocess.STDOUT
+                                   stderr=subprocess.DEVNULL,
                                    ).decode().strip()
 
 
 def get_revision(git_dir):
     try:
-        rev = get_cmd('git describe --tags', git_dir)
-        if not rev.startswith("fatal:"):
-            return rev
-
-        rev = get_cmd('git rev-parse HEAD', git_dir)
-        if not rev.startswith("fatal:"):
-            return rev
+        return get_cmd('git describe --tags --always --dirty', git_dir)
     except subprocess.CalledProcessError:
         pass