create_release.sh: use ATX markdown hdr instead of bold for changes and fixes

This allows having some formatting in FIXES.md, eg. for different minor
versions like that:

    #### 1.7.1
    - changes for 1.7.1
    - another fix in 1.7.1
    ### 1.7.2
    - fixes for 1.7.2

+ join the lines like beginning with spaces more inteligently