Fix more gutter close button alignment issues (#15233)

Follow-up of https://github.com/zed-industries/zed/pull/15178

* shows proper cursor on hovering a block that's over a git hunk
* show gutter buttons better when git hunks are on the same line
* show deleted hunks' gutter buttons better when git blame info is shown
in the gutter

Release Notes:

- N/A