Cleanup background fetch

[stefanhaller]

• PR Description

Simplify startBackgroundFetch

This code had a lot of logic that (fortunately) didn't work because it was buggy:

• it was supposed to wait for the auto-fetch delay before fetching for the first
  time in case we start with a repo that we had open in a previous session (i.e.
  that appears in the recent repos list). This code actually ran always, not
  just for known repos, because the IsNewRepo flag is only set later, after this
  function runs. Fortunately, the code didn't work, because time.After starts a
  timer but doesn't wait for it (to do that, it would have to be
  <-time.After).
• if the first fetch fails with error 128, it was supposed to show an error
  message and not start the background fetch loop. Fortunately, this didn't work
  because 1) it was guarded by isNew which is always false here, and 2) because
  git's error message in this case is actually "exit code: 128", not "exit
  status 128" (maybe this has changed in git at some point).

I find both of these undesirable. Whenever I open a repo I want an auto-fetch to
be triggered immediately to get my branch information up to date as quickly as
possible. And if the initial fetch fails (e.g. because one of my remotes is
offline or doesn't exist any more), then that's no reason not to start the
auto-fetch loop anyway.

So let's simplify the code to do what it did before, but with much fewer lines
of code.

See #4083.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for 62e31ef1	✅ 27.27%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (62e31ef)	Report Missing	Report Missing	Report Missing
Head commit (64cebfc)	50824	43825	86.23%

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#4084)	11	3	27.27%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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Footnotes

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[jesseduffield]

Looks good