fmt-merge-msg: remove custom string_list implementation
fmt-merge-msg: remove custom string_list implementation

This command uses a custom version of string list when it could
just as easily use the string_list API. Convert it to use string_list
and reduce the code size a bit.

Signed-off-by: Stephen Boyd <66c4b3fccf5786e0c6198a1e0f640664da3951b2@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git