commit: use FLEX_ARRAY in struct merge_remote_desc

commit: use FLEX_ARRAY in struct merge_remote_desc

Convert the name member of struct merge_remote_desc to a FLEX_ARRAY and
use FLEX_ALLOC_STR to build the struct.  This halves the number of
memory allocations, saves the storage for a pointer and avoids an
indirection when reading the name.

Suggested-by: Jeff King <redacted>
Signed-off-by: Rene Scharfe <redacted>
Reviewed-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>