Add custom git commands to dotfiles repo

Include git-branch-backup, git-fix-author, git-name-stash,
git-name-stash-apply, git-rebase-on, and git-rename-branch in a new
bin/ directory. Update install.sh to symlink them into ~/bin and
document them in the README.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

bin/git-fix-author

@@ -0,0 +1,51 @@
+#!/bin/bash
+
+OLD=""
+NEW_NAME=""
+NEW_EMAIL=""
+
+ARGS=$(getopt -o o:n:h -l "old:,new:,help" -n $0 -- "$@")
+
+[ $? -eq 0 ] || exit 1
+
+eval set -- "$ARGS"
+
+while true
+do
+    case "$1" in
+        -o|--old)
+            OLD=$2
+            shift 2
+            ;;
+        -n|--new)
+            NEW=$2
+            NEW_NAME=$(echo "$NEW" | awk -F'[<>]' '{print $1}' | sed 's/ *$//')
+            NEW_EMAIL=$(echo "$NEW" | awk -F'[<>]' '{print $2}')
+            shift 2
+            ;;
+        -h|--help)
+            echo "usage: git fix-author --old diz@cpan.org --new 'Mike Eldridge <mike.eldridge@iinteractive.com>' ref"
+            exit
+            ;;
+        --)
+            shift
+            break
+            ;;
+    esac
+done
+
+echo "OLD=\"$OLD\""
+echo "NEW_NAME=\"$NEW_NAME\""
+echo "NEW_EMAIL=\"$NEW_EMAIL\""
+
+git filter-branch --commit-filter "
+    if [ \"\$GIT_COMMITTER_EMAIL\" = \"$OLD\" ];
+    then
+        GIT_COMMITTER_NAME=\"$NEW_NAME\";
+        GIT_AUTHOR_NAME=\"$NEW_NAME\";
+        GIT_COMMITTER_EMAIL=\"$NEW_EMAIL\";
+        GIT_AUTHOR_EMAIL=\"$NEW_EMAIL\";
+        git commit-tree \"\$@\";
+    else
+        git commit-tree \"\$@\";
+    fi" $@