git-gutter:update-interval not work well with git-gutter:set-start-revision

When I set `git-gutter:set-start-revision` to `HEAD~1` to display the modifications introduced by the last commit, the icons showing line differences relative to HEAD~1 disappear after the interval specified by `git-gutter:update-interval`, provided that the interval is not zero. Perhaps the function `defsubst git-gutter:start-raw-diff-process (proc-buf original now)` should include a check for the `start-rev` parameter to address this behavior.