Fix 2-dot direct compare to use the right base commit (#24133) (#24150)

Backport #24133 by @jtran

For 2-dot direct compare, we should use the base commit in the title and
templates, as is used elsewhere, not the common ancestor which is used
for 3-dot compare. I believe that this change should have been included
in #22949.

Co-authored-by: Jonathan Tran <jonnytran@gmail.com>

routers/web/repo/compare.go

@@ -551,7 +551,11 @@ func ParseCompareInfo(ctx *context.Context) *CompareInfo {
 		ctx.ServerError("GetCompareInfo", err)
 		return nil
 	}
+	if ci.DirectComparison {
+		ctx.Data["BeforeCommitID"] = ci.CompareInfo.BaseCommitID
+	} else {
 		ctx.Data["BeforeCommitID"] = ci.CompareInfo.MergeBase
+	}
 
 	return ci
 }