fixed issue Merging two ways corrupts step_count #10492

[Darshit42]

this pr fixes issue #10492 ,changes are such that it adds up the values of tags

before fixing the issue tags such as step_count , parking:left, etc values were not adding up when lines were merged, this creates confusion to user on how merging works
merging two ways resulted in 3+4 => Error (values are not same) or 2+2 => 2 (values are not summed)

after this , when lines are merged it adds up the values of tags successfully
merging two ways resulted in 3+4 => 7

Screen-Recording.mp4

[Darshit42]

to anyone who's reading this , if i made any mistakes in raising this pr feel free to criticize it's my first time raising a pr in other repo besides mine 🙃