Fix IDE not opening diff correctly when switching to commit view

What does this MR do?

This MR fixes a bug in the Web IDE where switching to the commit tab does not load the diff immediately because it uses the file's new content as the "original".

Screenshots

Before (on master)	After (in this MR 2020-04-14)
20200414_ide_commit_pending_tab_before	20200414_ide_commit_pending_tab_fix

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

References

• Related to #199195 (closed)