Add Subsections and Subsection for Git-style section names - #436

Describe the pull request

Adds File.Subsections(name) and File.Subsection(name, sub) for Git config-style section names such as [forge "github.com"], so callers like git-lfs-server no longer cut prefixes and quotes off Section.Name() by hand.

for host, sec := range f.Subsections("forge") { ... }
f.Subsection("storage", "r2").Key("TYPE").SetValue("s3-presign") // adds [storage "r2"]

Both return the *Section of each header, so MapTo, %(name)s references and shadows work as usual. Section headers are read as Git reads them:

• [name "sub"] and the older [name.sub] both count, with a dotted sub lowercased. The name matches in any case. A dot in the name moves the rest into the sub, so [forge.a "b"] has sub a.b. Spaces and tabs are allowed between the parts but not around them. In a quoted sub, \" and \\ read as " and \, and a backslash before another character is dropped.
• Headers that don't split this way, such as [ forge "x"] or [forge "x" ], are skipped without an error.
• Subsections reads the sections each time it is ranged over.
• Subsection returns the section written as [name "sub"], or else the first match. If there is none, it returns a section that reads as empty, like Section does, and a change adds it with " and \ in the sub escaped, lowercased under the case-insensitive options.

Section names are parsed when they are read, not by the lexer, so parsing and stored section names are unchanged. That leaves three differences from Git, all documented:

• The name may have any character but a space, a tab or ", so [for_ge "x"] is read where Git rejects it.
• Headers spelled differently for the same sub, such as [forge "x"], [Forge "x"] and [forge.x], are separate sections, and Subsections yields the sub once for each, so callers can detect them. Headers spelled the same are one section, as they are for Section.
• With CaseInsensitive or CaseInsensitiveSections, the loader lowercases whole section names, so [forge "X"] and [forge "x"] are one section with sub x, where Git keeps them separate.

Link to the issue: n/a

Checklist

• I agree to follow the Code of Conduct by submitting this pull request.
• I have read and acknowledge the Contributing guide.
• I have added test cases to cover the new code.

AI usage

Written with Amp (Claude), reviewed by the author.