Update Merge Request > Changes > File filter bar
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# Context
https://gitlab.com/gitlab-org/gitlab/-/issues/19530 https://gitlab.com/gitlab-org/gitlab/-/issues/525169
Source Code is investigating adding a file tree to the Repository pages in https://gitlab.com/gitlab-org/gitlab/-/issues/525169. The file filter bar in Merge Request \> Changes tab currently looks like a search bar, but behaves like a filter. This design is not communicating accurately that the UI actually filters. This can be confusing to users.
# Design Recommendation
For both the Repository file tree, and Merge Request \> Changes tab filter, update the filter bar with a filter icon and appropriate placeholder text. The Repository file tree changes will be addressed in https://gitlab.com/gitlab-org/gitlab/-/issues/19530. This issue is to address the Merge Request \> Changes tab filter.
* Update the icon to a filter icon instead of a search icon
* Update the placeholder text

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