Check for backslashes in remote branch name

[ejoffe]

Fixes #332

commit-id:a7c1a3c3

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Stack:

• Check for backslashes in remote branch name #380 ⬅
• init : write repo and user config if they don't exist #379
• star logic : don't panic if star api returns error #378

⚠️ Part of a stack created by spr. Do not merge manually using the UI - doing so may have unexpected results.