perf(gitdiff): optimize inline diff highlighting using cache (#38706)

This pull request optimizes code diff highlighting in the PR Files
Changed view, which is a major CPU bottleneck and hot path in
production.

Previously, Gitea executed the expensive `DiffMatchPatch` algorithm
twice for every matching deleted/added line pair (once for the deleted
line, and once for the added line). We now run the diff algorithm once
and cache the resulting `DiffInline` on the `DiffLine` struct itself,
allowing the second line to render via a 0-cost cache lookup.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>

modules/charset/escape_status.go

@@ -10,14 +10,8 @@ type EscapeStatus struct {
 	HasAmbiguous bool
 }
 
-// Or combines two EscapeStatus structs into one representing the conjunction of the two
-func (status *EscapeStatus) Or(other *EscapeStatus) *EscapeStatus {
-	st := status
-	if status == nil {
-		st = &EscapeStatus{}
-	}
+func (st *EscapeStatus) Combine(other *EscapeStatus) {
 	st.Escaped = st.Escaped || other.Escaped
 	st.HasAmbiguous = st.HasAmbiguous || other.HasAmbiguous
 	st.HasInvisible = st.HasInvisible || other.HasInvisible
-	return st
 }