Only call git-rerere if $GIT_DIR/rr-cache exists.

Johannes noticed that git-rerere depends on Digest.pm, and if
one does not use the command, one can live without it.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-am.sh

@@ -88,7 +88,10 @@ fall_back_3way () {
     # saying that we reverted all those changes.
 
     git-merge-resolve $orig_tree -- HEAD $his_tree || {
-	    git-rerere
+	    if test -d "$GIT_DIR/rr-cache"
+	    then
+		git-rerere
+	    fi
 	    echo Failed to merge in the changes.
 	    exit 1
     }

git-commit.sh

@@ -638,7 +638,10 @@ else
 fi
 ret="$?"
 rm -f "$GIT_DIR/COMMIT_MSG" "$GIT_DIR/COMMIT_EDITMSG"
-git-rerere
+if test -d "$GIT_DIR/rr-cache"
+then
+	git-rerere
+fi
 
 if test -x "$GIT_DIR"/hooks/post-commit && test "$ret" = 0
 then

git-merge.sh

@@ -309,6 +309,9 @@ Conflicts:
 		sed -e 's/^[^	]*	/	/' |
 		uniq
 	} >>"$GIT_DIR/MERGE_MSG"
-	git rerere
+	if test -d "$GIT_DIR/rr-cache"
+	then
+		git-rerere
+	fi
 	die "Automatic merge failed; fix up by hand"
 fi