Backpatch addition of .git-blame-ignore-revs

Backpatch addition of .git-blame-ignore-revs

This makes it more convenient for git config to contain the
blame.ignoreRevsFile setting; otherwise current git versions complain if
the file is not present.

I constructed the file for each branch by scraping the file in branch
master for commits that appear in that branch.  Because a few additional
pgindent commits have been added to the list in master since the list
was first created, this also propagates those to branches 14 and 15
where the file already existed.  Also, some entries appear to have been
made using author-date rather than committer-date in the format string,
so some timestamps are changed.  Also remove bogus whitespace in the
suggested `git log` format string.

Backpatch to all supported branches.

Discussion: https://postgr.es/m/20220711163138.o72evdeus5f5yy5z@alvherre.pgsql