fix(git): Silence "file not found" logs

When trying to open a file in Git for reading which does not (yet)
exist, Git will print an error message to its stderr. This currently
gets forwarded as ERROR level log message. However, since these errors
are also turned into FileNotFoundError exceptions for proper downstream
handling, the logs are usually harmless (and, in fact, quite
misleading).

This commit changes errors that occur when trying to open a file for
reading to DEBUG level.

capellambse/filehandler/git.py

@@ -836,7 +836,9 @@ def __is_lfs(self, path: pathlib.PurePosixPath) -> bool:
         return True
 
     def __open_from_index(self, filename: pathlib.PurePosixPath) -> bytes:
-        return self._git("cat-file", "blob", f"{self.revision}:{filename}")
+        return self._git(
+            "cat-file", "blob", f"{self.revision}:{filename}", silent=True
+        )
 
     def __open_from_lfs(self, filename: pathlib.PurePosixPath) -> bytes:
         lfsinfo = self.__open_from_index(filename)