Add .git-blame-ignore-revs (esp8266#8464)

Ignore mass-reformatting commits
Include the clang-format, and the previous astyle

Based on the idea from black
https://black.readthedocs.io/en/stable/guides/introducing_black_to_your_project.html#avoiding-ruining-git-blame

To be used with git CLI when exploring some specific file history
```
$ git blame --ignore-revs-file .git-blame-ignore-revs -- file.cpp
```

Or, by modifying the config to apply globally
```
$ git config blame.ignoreRevsFile .git-blame-ignore-revs
```
(note that the by default config will apply only for the current repository)

Author: mcspr

.git-blame-ignore-revs

@@ -0,0 +1,4 @@
+9bdcd4f36a2e5285267b69b17e8fc26482dc1c72
+eea9999dc5eaf464a432f77d5b65269f9baf198d
+98125f88605cd7e46e9be4e1b3ad0600dd5d2b51
+91213321df5d49622e7d03426da8366197fe0db0