Respect branch info for relative links (#28909) (#28922)

Backport #28909 by @KN4CK3R

Fix #28904

Co-authored-by: KN4CK3R <admin@oldschoolhack.me>

modules/markup/markdown/goldmark.go

@@ -137,6 +137,8 @@ func (g *ASTTransformer) Transform(node *ast.Document, reader text.Reader, pc pa
 				var base string
 				if ctx.IsWiki {
 					base = ctx.Links.WikiLink()
+				} else if ctx.Links.HasBranchInfo() {
+					base = ctx.Links.SrcLink()
 				} else {
 					base = ctx.Links.Base
 				}

modules/markup/markdown/markdown_test.go

@@ -765,7 +765,7 @@ space</p>
 			Expected: `<p>space @mention-user<br/>
 /just/a/path.bin<br/>
 <a href="https://example.com/file.bin" rel="nofollow">https://example.com/file.bin</a><br/>
-<a href="/user/repo/file.bin" rel="nofollow">local link</a><br/>
+<a href="/user/repo/src/branch/main/file.bin" rel="nofollow">local link</a><br/>
 <a href="https://example.com" rel="nofollow">remote link</a><br/>
 <a href="/user/repo/src/branch/main/file.bin" rel="nofollow">local link</a><br/>
 <a href="https://example.com" rel="nofollow">remote link</a><br/>
@@ -878,7 +878,7 @@ space</p>
 			Expected: `<p>space @mention-user<br/>
 /just/a/path.bin<br/>
 <a href="https://example.com/file.bin" rel="nofollow">https://example.com/file.bin</a><br/>
-<a href="/user/repo/file.bin" rel="nofollow">local link</a><br/>
+<a href="/user/repo/src/branch/main/sub/folder/file.bin" rel="nofollow">local link</a><br/>
 <a href="https://example.com" rel="nofollow">remote link</a><br/>
 <a href="/user/repo/src/branch/main/sub/folder/file.bin" rel="nofollow">local link</a><br/>
 <a href="https://example.com" rel="nofollow">remote link</a><br/>