Don't default the base repo for pull requests to "origin"

[stefanhaller]

When no base repository is configured using gh, we have several heuristics for choosing a likely base repo before prompting the user. One of these was that if the origin remote exists, we'd choose that. This may be the right choice in some cases, but in many others it's not; for example, in the common scenario of a forking setup where the upstream repo's remote is called after its owner, and the fork remote is called origin, we would choose origin as the base repo, which results in not showing any PRs. Don't do that, and instead prompt the user for the base repo in this case.

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 0 duplication

Metric	Results
Duplication	0

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🟢 Coverage 0.00% diff coverage

Metric	Results
Coverage variation	Report missing for 9cd5fb81
Diff coverage	✅ 0.00% diff coverage

View coverage diff in Codacy

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (9cd5fb8)	Report Missing	Report Missing	Report Missing
Head commit (4218f97)	61937	53901	87.03%

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 (#5508)	2	0	0.00%

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%

¹ Codacy didn't receive coverage data for the commit, or there was an error processing the received data. Check your integration for errors and validate that your coverage setup is correct.

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