Fix 500 error when comparing branches across fork repositories
## Problem
The compare API returns a 500 Internal Server Error when comparing
branches where the head commit exists only in the fork repository.
## Cause
The API was using the base repository's GitRepo and repository context
when converting commits. This fails when the commit does not exist in
the base repository, resulting in a "fatal: bad object" error.
## Solution
Use the head repository and HeadGitRepo when available to ensure commits
are resolved in the correct repository context.
## Result
* Fixes "fatal: bad object" error
* Enables proper comparison between base and fork repositories
* Prevents 500 Internal Server Error
Fixes#37168
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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
80585adab4