Default upstream refspec to master

[smashwilson]

The first scheduled run of our schema-up action failed on master:

ℹ  info      Fetching the latest GraphQL schema changes.
ℹ  info      Committing schema changes.
ℹ  info      Re-running relay compiler.
ℹ  info      Relay output:

Writing js
Unchanged: 62 files
ℹ  info      Generated relay files are unchanged.
✖  fatal     TypeError: Cannot read property 'replace' of undefined 
    at Toolkit.run (/index.js:45:40)
    at <anonymous>
    at process._tickCallback (internal/process/next_tick.js:189:7)

### FAILED Update schema 01:01:22Z (21.425s)

I'd assumed that toolkit.context.ref would be populated with the repository's default branch when run from a schedule trigger, but this is not the case:

The branch or tag ref that triggered the workflow. For example, refs/heads/feature-branch-1. If neither a branch or tag is available for the event type, the variable will not exist.

The easiest fix here is to default to master.

[smashwilson]

I'm also changing the scheduled weekday to Tuesday so we'll have a chance to see if it works tonight rather than waiting an entire week.

[codecov]

Codecov Report

Merging #2125 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2125      +/-   ##
==========================================
- Coverage   92.66%   92.65%   -0.01%     
==========================================
  Files         207      207              
  Lines       12045    12045              
  Branches     1764     1764              
==========================================
- Hits        11161    11160       -1     
- Misses        884      885       +1

Impacted Files	Coverage Δ
lib/atom/gutter.js	89.74% <0%> (-2.57%)	⬇️

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[smashwilson]

Self-merging because there's no way to know if this will work or not until it's on master 🎶