Save deprecate-email script

For many years this was in a Google Keep note, so really good
to properly version control it now.

bin/deprecate-email

@@ -0,0 +1,38 @@
+#!/usr/bin/env dash
+set -eu
+
+# deprecate-email:
+# Sometimes I remove email addresses from my GitHub profile. This makes the contributions in my profile disappear as the email address is no longer associated with my account. This script replaces commits with input email with current git config.
+
+if [ -z "$1" ]; then
+  echo "Error: old_email argument is required."
+  exit 1
+fi
+
+old_email="$1"
+new_email="$(git config user.email)"
+new_name="$(git config user.name)"
+
+# Replacing author and committer emails has to be done separately to avoid unintentional overwrites
+# e.g. in commits with multiple authors.
+(export FILTER_BRANCH_SQUELCH_WARNING=1 && \
+echo "1/2 Overwriting committer" && \
+git filter-branch -f --commit-filter "
+	if [ \"\$GIT_COMMITTER_EMAIL\" = \"$old_email\" ];
+	then
+		GIT_COMMITTER_NAME=\"$new_name\";
+		GIT_COMMITTER_EMAIL=\"$new_email\";
+		git commit-tree \"\$@\";
+	else
+		git commit-tree \"\$@\";
+	fi" HEAD && \
+echo "2/2 Overwriting author" && \
+git filter-branch -f --commit-filter "
+	if [ \"\$GIT_AUTHOR_EMAIL\" = \"$old_email\" ];
+	then
+		GIT_AUTHOR_NAME=\"$new_name\";
+		GIT_AUTHOR_EMAIL=\"$new_email\";
+		git commit-tree \"\$@\";
+	else
+		git commit-tree \"\$@\";
+	fi" HEAD)