Add Bash completion file

Author: Jorenar

completion.bash

@@ -0,0 +1,22 @@
+__git_fmt_diff_options="
+    --cached
+    --staged
+    --color
+    --no-color
+    --config
+    --warn-risky-stderr
+"
+
+_git_fmt_diff () {
+    __git_has_doubledash && return
+
+    # shellcheck disable=SC2154
+    case "$cur" in
+        --*)
+            __gitcomp "$__git_fmt_diff_options"
+            return
+            ;;
+    esac
+
+    __git_complete_revlist_file
+}