Simplify escaping of pattern and fix match_prev_cmd strategy

Maybe this is also a fix for #247, #248 and #258. Supersedes #267.

Testcase:
Using match_prev_cmd strategy and with these lines in history:
echo '1^'
echo '2^'
echo '1^'

type:
echo       (unexpected suggestion echo '1^' instead of echo '2^')
echo '1^1  (wrong suggestion echo '1^1echo '1^')
echo '1^#  (error "bad math expression")

Makefile

@@ -3,7 +3,6 @@ SRC_DIR    := ./src
 SRC_FILES := \
 	$(SRC_DIR)/setup.zsh \
 	$(SRC_DIR)/config.zsh \
-	$(SRC_DIR)/util.zsh \
 	$(SRC_DIR)/features.zsh \
 	$(SRC_DIR)/bind.zsh \
 	$(SRC_DIR)/highlight.zsh \