Apps.ListRepos/Apps.ListUserRepos: Add mercy-preview + nebula-preview headers

[andygrunwald]

What type of PR is this?

feature

What this PR does / why we need it:

• mercy-preview: For receiving repository topics
• nebula-preview: For visibility information

When calling Apps.ListRepos or Apps.ListUserRepos, the field visibility and topic are always nil.

The reason: Missing headers for developers preview API.

Special notes for your reviewer:

This applies the same fix as in #1780

Does this PR introduce a user-facing change?:

None

Additional documentation e.g., usage docs, etc.:

• API changes to support internal repository visibility (2019-12-03)

[codecov]

Codecov Report

Merging #1806 (b535059) into master (5c87615) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@ Coverage Diff @@## master #1806 +/- ##
=======================================
Coverage 97.59% 97.59% =======================================
Files 100 100 Lines 6521 6525 +4 =======================================
+ Hits 6364 6368 +4 
Misses 85 85 Partials 72 72 

Impacted Files	Coverage Δ
github/apps_installation.go	91.30% <100.00%> (+0.82%)	⬆️

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[gmlewis]

Thank you, @andygrunwald !
I've gone through the GitHub v3 API docs and can't find any mention of the nebula-preview custom header... why is that one needed? EDIT: Ah! I see you already covered this in your description... awesome. Thank you!

Otherwise, LGTM.

Awaiting second LGTM before merging.

[gmlewis]

Never mind my last comment. 😂 See edit above.

[wesleimp]

💙

[andygrunwald]

@gmlewis@wesleimp Good to merge? Anything I can do to support?

[gmlewis]

Thank you, @wesleimp !
Merging.