histedit: Only parse old=new for fc -s

Before (erroneously replaced):
	$ a=b set
	...
	$ fc a=b
	8
	,
	b=b set
After (used as search string):
	$ fc a=b
	8
	,
	a=b set

Reported-by: Harald van Dijk <harald@gigawatt.nl>
Reported-in: https://marc.info/?l=dash&m=154707728009743&w=2
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
1 file changed