replace memset with string-list initializers

replace memset with string-list initializers

Using memset and then manually setting values of the string-list
members is not future proof as the internal representation of
string-list may change any time.
Use `string_list_init()` or STRING_LIST_INIT_* macros instead of
memset.

Signed-off-by: Tanay Abhra <redacted>
Reviewed-by: Matthieu Moy <redacted>
Signed-off-by: Junio C Hamano <redacted>