git-gui: Honor rerere.enabled configuration option

git-gui: Honor rerere.enabled configuration option

Recently in git.git change b4372ef136 Johannes Schindelin taught
git-commit.sh to invoke (or skip) calling git-rerere based upon
the rerere.enabled configuration setting:

  So, check the config variable "rerere.enabled". If it is set
  to "false" explicitely, do not activate rerere, even if
  .git/rr-cache exists. This should help when you want to disable
  rerere temporarily.

  If "rerere.enabled" is not set at all, fall back to detection
  of the directory .git/rr-cache.

We now do the same logic in git-gui's own commit implementation.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>