Support Bash completions for short option group in journalctl (#40214)

Support Bash completions for short option group in journalctl (#40214)

Currently, the Bash completions for journalctl tries to match the
previous word _**exactly**_, which leads to the following issue:
`journalctl -u dock` correctly auto completes to `journalctl -u
docker.service`, but `journalctl -eu` provides no completions at all,
which is a shame since I never use the `-u` option alone (almost always
`-eu` or `-efu`, I wish the `-e` option was the default but I digress).

The proposed solution is to assume words that start with only a single
dash and consist of only letters are short option groups and handle them
as if the previous word was the short option using the last character,
e.g. `-efu` -> `-u`.