Full-file syntax highlighting for diff pages

[dfirebaugh]

Fix #33358, fix #21970

This adds a step in the GitDiff that does syntax highlighting for the entire file and then only references lines from that syntax highlighted code. This allows things like multi-line comments to be syntax highlighted correctly.

[wxiaoguang]

HTML encoding should be correctly handled, for example: <>

See new comment below #33766 (comment)

[wxiaoguang]

Thank you very much. I can understand most of changes now. The idea is feasible, while there are still some details:

1. Some of the old tests should be kept, for example: diff.Text = "OC", I do not see why it should be removed. It is used to make sure the recoverOneDiff works correctly.
2. Some of the old tests do not fit the new code:
   • If a test doesn't fit (the case won't happen), feel free to remove it.
   • New tests should cover the new behavior since the behavior of diffWithHighlight has changed, it only accepts HTML content
3. Some variable types should be changed to match the new behavior, use template.HTML
4. Some edge cases, for example, what if a file is large (Mega-bytes)? It would bloat the Gitea's memory and make it OOM.
   • I think we need to have a limit, if a file is too large, fall back to the "hunk block" diff but not the "full file" diff.

[silverwind]

I think we need to have a limit, if a file is too large, fall back to the "hunk block" diff but not the "full file" diff.

Sounds acceptable to have a certain byte limit above which to fall back to hunk-based diff. Maybe 1-5MB or so.

[lunny]

When expend more codes, it result in 500

Render failed, failed to render template: repo/diff/blob_excerpt, error: template error: builtin(static):repo/diff/blob_excerpt:83:20 : executing "repo/diff/blob_excerpt" at <$.section.GetComputedInlineDiffFor>: error calling GetComputedInlineDiffFor: runtime error: invalid memory address or nil pointer dereference
----------------------------------------------------------------------
		{{$inlineDiff := $.section.GetComputedInlineDiffFor $line ctx.Locale}}
		                  ^
----------------------------------------------------------------------

[wxiaoguang]

When expend more codes, it result in 500

Fixed in b2dbc78 and added more tests

[wxiaoguang]

Follow up fix: Fix material icon & diff highlight #33844