<branch>:HEAD is not supported for option "-b"

[LumnitzF]

Using the HEAD commit for a specific branch currently leads to an exception:

java.lang.NumberFormatException: For input string: "HEAD"
            at java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
            at java.base/java.lang.Long.parseLong(Long.java:692)
            at java.base/java.lang.Long.parseLong(Long.java:817)
            at com.teamscale.buildbreaker.BuildBreaker.isTeamscaleAnalysisFinished(BuildBreaker.java:407)
            at com.teamscale.buildbreaker.BuildBreaker.call(BuildBreaker.java:291)
            at com.teamscale.buildbreaker.BuildBreaker.call(BuildBreaker.java:52)
            at picocli.CommandLine.executeUserObject(CommandLine.java:1933)
            at picocli.CommandLine.access$1200(CommandLine.java:145)
            at picocli.CommandLine$RunLast.executeUserObjectOfLastSubcommandWithSameParent(CommandLine.java:2332)
            at picocli.CommandLine$RunLast.handle(CommandLine.java:2326)
            at picocli.CommandLine$RunLast.handle(CommandLine.java:2291)
            at picocli.CommandLine$AbstractParseResultHandler.execute(CommandLine.java:2159)
            at picocli.CommandLine.execute(CommandLine.java:2058)
            at com.teamscale.buildbreaker.BuildBreaker.main(BuildBreaker.java:273)

As this is especially mentioned in the README it should:
a) be supported, or
b) removed from the README

IMO it doesn't really make sense to use HEAD for the timestamp, because the HEAD commit may point to a totally different commit due to, e.g., a rollback