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>

lib/commit.tcl

@@ -331,7 +331,12 @@ A rescan will be automatically started now.
 
 	# -- Let rerere do its thing.
 	#
-	if {[file isdirectory [gitdir rr-cache]]} {
+	if {[get_config rerere.enabled] eq {}} {
+		set rerere [file isdirectory [gitdir rr-cache]]
+	} else {
+		set rerere [is_config_true rerere.enabled]
+	}
+	if {$rerere} {
 		catch {git rerere}
 	}