Enable "git rerere" by the config variable rerere.enabled

Enable "git rerere" by the config variable rerere.enabled

Earlier, "git rerere" was enabled by creating the directory
.git/rr-cache.  That is definitely not in line with most other
features, which are enabled by a config variable.

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.

[jc: with minimum tweaks]

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>