Support function renamings

[marco-c]

Currently, a renamed function is considered as a new function.
For example, when we collect the bugs that occurred in the same function (

bugbug/scripts/past_bugs_by_unit.py

Line 99 in bdb91ce

past_regressions_by_function[path][f[0]].extend(

), we use the function name. This means we lose bugs whenever a function is renamed.