Handle spaces in project paths for git and package commands

[mikeger]

• add projectBasePathWithSpaces regression test that renames the fixture .xcodeproj to include a space and verifies selective testing still returns the expected targets
• introduce String.shellQuoted helper and use it to wrap every interpolated path/branch sent to cd, git, swift package, and similar shell invocations so commands remain valid with spaces or quotes in
  paths
• ensure SwiftPM manifest parsing and git changeset detection now work reliably for projects whose names contain whitespace

[mikeger]

Resolves #80