How to ignore files during merging to master in git

You have Branch A and you made many changes from the master but you do not want FileA to go to Master during merge even you change it. 

How to do it in git ?

Here git-update-index - Register file contents in the working tree to the index.

git update-index --assume-unchanged <PATH_OF_THE_FILE>

Example:-

git update-index --assume-unchanged somelocation/pom.xml