StringIndexOutOfBoundsException in shell-jline2 completion

[remkop]

Steps to reproduce:
Start the example with:

java -cp picocli-3.9.0-SNAPSHOT.jar;picocli-shell-jline2-3.9.0-SNAPSHOT.jar;jline-2.14.6.jar;picocli-shell-jline2-3.9.0-SNAPSHOT-tests.jar picocli.shell.jline2.example.Example

type this command in the shell:

cmd --timeUnit=

Bring the cursor back one position so it is in front of the = character, and hit <TAB>. This will generate the following error:

prompt> cmd --timeUnitjava.lang.StringIndexOutOfBoundsException: String index out of range: -1
        at java.lang.String.substring(Unknown Source)
        at picocli.AutoComplete.complete(AutoComplete.java:600)
        at picocli.shell.jline2.PicocliJLineCompleter.complete(PicocliJLineCompleter.java:48)
        at jline.console.ConsoleReader.complete(ConsoleReader.java:3312)
        at jline.console.ConsoleReader.readLine(ConsoleReader.java:2653)
        at jline.console.ConsoleReader.readLine(ConsoleReader.java:2378)
        at jline.console.ConsoleReader.readLine(ConsoleReader.java:2366)
        at jline.console.ConsoleReader.readLine(ConsoleReader.java:2354)
        at picocli.shell.jline2.example.Example.main(Example.java:97)

[remkop]

Fixed in commit e63d231.