metadata: add .git-blame-ignore-revs

Someone mentioned this on IRC so I thought I would try it out with a
few commits that are pure code style fixes.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230403134920.2132362-7-alex.bennee@linaro.org>

.git-blame-ignore-revs

+#
+# List of code-formatting clean ups the git blame can ignore
+#
+#   git blame --ignore-revs-file .git-blame-ignore-revs
+#
+# or
+#
+#   git config blame.ignoreRevsFile .git-blame-ignore-revs
+#
+
+# gdbstub: clean-up indents
+ad9e4585b3c7425759d3eea697afbca71d2c2082
+
+# e1000e: fix code style
+0eadd56bf53ab196a16d492d7dd31c62e1c24c32
+
+# target/riscv: coding style fixes
+8c7feddddd9218b407792120bcfda0347ed16205
+
+# replace TABs with spaces
+48805df9c22a0700fba4b3b548fafaa21726ca68