LUCENE-9570: add parial reformatting commit reference to .git-blame-ignore-revs [thanks rmuir] and add a note to git.txt aiding in making this a persistent local option.

.git-blame-ignore-revs

@@ -0,0 +1,2 @@
+# LUCENE-9570 [google java formatter applied]
+6faa4f98e04

help/git.txt

@@ -1,3 +1,6 @@
+git
+===
+
 A list of resources that may be helpful for those learning git:
 
  - Dawid's task-oriented help:
@@ -12,3 +15,13 @@ A list of resources that may be helpful for those learning git:
 
  - git workflows, their pros and cons:
    https://help.github.com/articles/what-is-a-good-git-workflow/
+
+
+git blame
+=========
+
+Git blame shows the last revision that touched a line in a given file. Certain
+reformatting commits may stand in the way here. Please issue the following
+command so that git ignores those irrelevant commits:
+
+git config blame.ignoreRevsFile .git-blame-ignore-revs