Feature request: automatically read .git-blame-ignore-revs or allow global optional config

I have many repos where I use .git-blame-ignore-revs,
but I cannot set it globally because then I get
```
fatal: could not open object name list: .git-blame-ignore-revs
```
so please make it either the default for `git blame` to check that file,
or add a "blame: ignoreMissingFile: true" option so I an set
```
[blame]
  ignoreRevsFile = .git-blame-ignore-revs
  ignoreMissingFile: true
```
and can use this feature without constantly having to think about it