Commit Preview tweaks

[simurai]

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Description of the Change

Here some tweaks carried over from #1767 (comment).

1. Tab title

Change the tab title to "Staged Changes".

Before	After

This makes it the same as the title in the staging view:

2. Move the "See All Staged Changes" button

So that it's part of the "Staged Changes" box.

Before	After

Note: This is currently just a visual hack. Under the hood, the button is still part of CommitView.

Alternate Designs

This is an alternative to #1767.

Benefits

Less confusion around clicking a "See All Staged Changes" button, but then opening a "Commit Preview" pane.

Possible Drawbacks

None, except maybe refactor all occurrences of CommitPreview.

Applicable Issues

Follow-up for #1767.

Metrics

N/A

Tests

N/A

Documentation

Documentation is tracked here #1791

Release Notes

N/A

User Experience Research (Optional)

I think the change from calling the button "Commit Preview" to "See All Staged Changes" comes from doing some user research. This PR takes it a bit further.

[coveralls]

Coverage increased (+0.06%) to 85.572% when pulling a465a67 on sm/commit-preview-tweaks into cc13510 on master.

[simurai]

Here some more options that aren't part of this PR yet, but could still be considered.

3. Remove the toggling

Is the "Hide All Staged Changes" really needed? Since the pane a pending pane, it will be replaced by another file. Or users can always close the pane with the X or cmd-w. If a user clicks on a single file above, we also don't have a "hide file" toggle.

4. Remove "Staged Changes for" prefix

In FilePatchViews, the header still says "Staged Changes for ...". Should we remove it here too? To keep it consistent with the "Commit Preview/Staged Changes" pane.

Before	After

[codecov]

Codecov Report

Merging #1805 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1805      +/-   ##
==========================================
- Coverage   91.15%   91.14%   -0.02%     
==========================================
  Files         196      196              
  Lines       10745    10745              
  Branches     1575     1575              
==========================================
- Hits         9795     9793       -2     
- Misses        950      952       +2

Impacted Files	Coverage Δ
lib/items/commit-preview-item.js	100% <100%> (ø)	⬆️
lib/git-shell-out-strategy.js	87.52% <0%> (-0.36%)	⬇️

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