Add .git-blame-ignore-revs

Add a `.git-blame-ignore-revs` file which can be used to skip specified
commits when blaming, this is useful to ignore formatting commits, like
clang-format #4790.

Github blame automatically supports this file format, as well as other
third party tools. Git itself needs to be told about the file name to
work, the following command will add it to the current git repo. `git
config blame.ignoreRevsFile .git-blame-ignore-revs`, alternatively one
has to specify it with every blame. `git blame --ignore-revs-file
.git-blame-ignore-revs search.cpp`

Supported since git 2.23.

closes #4969

No functional change

.git-blame-ignore-revs

@@ -0,0 +1,7 @@
+# .git-blame-ignore-revs
+# Ignore commit which added clang-format
+2d0237db3f0e596fb06e3ffbadba84dcc4e018f6
+
+# Post commit formatting fixes
+0fca5605fa2e5e7240fde5e1aae50952b2612231
+08ed4c90db31959521b7ef3186c026edd1e90307