[proposal] Custom git diff rendering for binary files

[HarvsG]

Currently gitea only shows git diffs for text files, and can't for binary files.

git diff has native support for custom drivers for converting binary files to text in order to display a diff.

Since gitea just runs the git diff command and parses stdout we can leverage this

gitea/services/gitdiff/gitdiff.go

Line 717 in 9542b73

cmd = exec.CommandContext(ctx, git.GitExecutable, diffArgs...)

By customising git config --global --edit and adding:

[core]
        quotePath = false
        attributesfile = /root/gitea/custom/git/.gitattributes
[diff "pandoc"]
     textconv=pandoc --to=markdown
     prompt = false

Which is equivalent to running:

git config --global core.attributesfile /root/gitea/custom/git/.gitattributes
git config --global diff.pandoc.textconv "pandoc --to=markdown"
git config --global diff.pandoc.prompt false

And then creating the file /root/gitea/custom/git/.gitattributes with contents:

*.docx diff=pandoc

We get a much better outpjut:

The proposal

I propse that we allow users to implement this in an easier way, this could be done in a number of ways.

1. Allow the user to customise their git config --global file - default location being /root/.gitconfig
   1. by using the includes field with value $GITEA_CUSTOM/git/config and allowing the user to configure that as they please
   2. or by creating a new section in app.ini called something like [git.config] and allow the user to build a git config from the entries there - this may require translation between .ini syntax and whatever syntax git config uses
2. Expand the external renderer section with new fields CUSTOM_DIFF and TEXTCONV_DIFF_COMMAND and use that to autopopulate /root/.gitconfig and a .gitattributes file

Note 1.a and 1.b generailse to allow the user to configure all of the git config options, 2 is specific to the rendering of diffs

[HarvsG]

Currently git config is run here in the checkAndSetConfig() command

gitea/modules/git/git.go

Lines 159 to 161 in ae3cfa8

	if _, stderr, err = process.GetManager().Exec(fmt.Sprintf("git.Init(set %s)", key), "git", "config", "--global", key, defaultValue); err != nil {
	return fmt.Errorf("Failed to set git %s(%s): %s", key, err, stderr)
	}

here (not sure this one is relevant)

gitea/services/pull/merge.go

Lines 159 to 168 in c52d48a

	var gitConfigCommand func() *git.Command
	if version.Compare(binVersion, "1.8.0", ">=") {
	gitConfigCommand = func() *git.Command {
	return git.NewCommand("config", "--local")
	}
	} else {
	gitConfigCommand = func() *git.Command {
	return git.NewCommand("config")
	}
	}

and here (also probably not relevant)

gitea/integrations/gitea-repositories-meta/user2/repo1.wiki.git/hooks/update.sample

Lines 45 to 50 in 801843b

	# --- Config
	allowunannotated=$(git config --bool hooks.allowunannotated)
	allowdeletebranch=$(git config --bool hooks.allowdeletebranch)
	denycreatebranch=$(git config --bool hooks.denycreatebranch)
	allowdeletetag=$(git config --bool hooks.allowdeletetag)
	allowmodifytag=$(git config --bool hooks.allowmodifytag)