git-flow-log: Remove bashisms

Do not use `[[`, because that is a bashism, and breaks `git flow log` when running under a /bin/sh that is not bash (for example, on Debian). Fixes [Debian #765520](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=765520).

Reported-by: Paul van Tilburg <paulvt@debian.org>
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>