[shed] Fix issues with multiple expressions

[WolfgangDrescher]

This PR fixes an error introduced in 9b16d0a when shed is used with multiple expressions at once:

input.krn

**kern	**kern	**kern
*clefC4	*clefC3	*clefC1
1c	1e	1g
==	==	==
*-	*-	*-

command:

cat input.krn | shed -e "s/^clefC1/clefG2/I s/^clefC3/clefG2/I s/^clefC4/clefG2/I"

output:

**kern	**kern	**kern
*clefC4	*clefC3	*clefG2
1c	1e	1g
==	==	==
*-	*-	*-
**kern	**kern	**kern
*clefC4	*clefG2	*clefG2
1c	1e	1g
==	==	==
*-	*-	*-
**kern	**kern	**kern
*clefG2	*clefG2	*clefG2
1c	1e	1g
==	==	==
*-	*-	*-