Use case-insensitive matching for Git error message "Not a valid object name"

Summary

Git is changing the capitalization of its fatal: Not a valid object name error message to lowercase (fatal: not a valid object name) to follow Git's CodingGuidelines, which state that error messages should begin with a lowercase letter.

This change is under review on the Git mailing list: https://lore.kernel.org/git/[email protected]

Problem

Harness currently uses case-sensitive string matching to detect this Git error, which will break when Git ships the updated message.

Affected files

• git/api/merge.go — `strings.HasPrefix(msg, "fatal: Not a valid object name")`
• git/api/tree.go — `strings.Contains(...)` with case-sensitive match

Suggested fix

Use case-insensitive comparison, for example: ```go strings.Contains(strings.ToLower(msg), "not a valid object name") ```

This will work with both the current and future Git versions.

Timeline

The Git change is currently under review and will likely be included in a future Git release. Fixing the matching now ensures forward compatibility.