Escape branch names in push instructions

What does this MR do and why?

Related to #354932 (closed)

It was possible to setup a project with a default branch name that could cause trouble for someone who carelessly copy-pasted the push instructions. This MR escapes the branch name.

The backslash escapes aren't the prettiest but I felt like using shellescape for this edge case was better than building our custom logic that would quote the string.

Screenshots or screen recordings

With a default branch name of ;rm -rf /

Before (repro on .com)

After (GDK)

How to set up and validate locally

1. Create a group
2. Set the default branch name to something like ;rm -rf / or anything with shell special characters in Settings > Repository
3. Create an empty project and observe the escaped special characters

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.