docs(git): add subdirectory example

[mkniewallner]

Summary

Closes #10889.

As kinda hinted in the issue, would adding a --subdirectory flag to uv add make sense? This would IMO be more discoverable.

Test Plan

Ran the command added to the documentation:

$ uv add git+https://github.com/langchain-ai/langchain#subdirectory=libs/langchain
Resolved 40 packages in 203ms
   Built langchain @ git+https://github.com/langchain-ai/langchain@f2ea62f63209130bfc56b1fe7d0fa7c299bbf352#subdirectory=libs/langchain
Prepared 19 packages in 837ms
Installed 37 packages in 14ms
[...]

[charliermarsh]

Nice, thanks.

[mickvangelderen]

As kinda hinted in the issue, would adding a --subdirectory flag to uv add make sense? This would IMO be more discoverable.

@mkniewallner You will have to refactor how unnamed package name resolution works. Currently the resolution works by checking out an URL which accepts this #subdirectory=<subdirectory> fragment.

The code here 8a6fac0 does the easy part, passing around a subdirectory parameter, but you also need to figure out how to change this

uv/crates/uv/src/commands/project/add.rs

Lines 256 to 357 in e3a0988

	// Resolve any unnamed requirements.
	let requirements = {
	// Partition the requirements into named and unnamed requirements.
	let (mut requirements, unnamed): (Vec<_>, Vec<_>) = requirements
	.into_iter()
	.map(|spec| {
	augment_requirement(
	spec.requirement,
	rev.as_deref(),
	tag.as_deref(),
	branch.as_deref(),
	)
	})
	.partition_map(|requirement| match requirement {
	UnresolvedRequirement::Named(requirement) => itertools::Either::Left(requirement),
	UnresolvedRequirement::Unnamed(requirement) => {
	itertools::Either::Right(requirement)
	}
	});
	// Resolve any unnamed requirements.
	if !unnamed.is_empty() {
	// TODO(charlie): These are all default values. We should consider whether we want to
	// make them optional on the downstream APIs.
	let build_constraints = Constraints::default();
	let build_hasher = HashStrategy::default();
	let hasher = HashStrategy::default();
	let sources = SourceStrategy::Enabled;
	// Add all authenticated sources to the cache.
	for index in settings.index_locations.allowed_indexes() {
	if let Some(credentials) = index.credentials() {
	uv_auth::store_credentials(index.raw_url(), credentials);
	}
	}
	// Initialize the registry client.
	let client = RegistryClientBuilder::try_from(client_builder)?
	.index_urls(settings.index_locations.index_urls())
	.index_strategy(settings.index_strategy)
	.markers(target.interpreter().markers())
	.platform(target.interpreter().platform())
	.build();
	// Determine whether to enable build isolation.
	let environment;
	let build_isolation = if settings.no_build_isolation {
	environment = PythonEnvironment::from_interpreter(target.interpreter().clone());
	BuildIsolation::Shared(&environment)
	} else if settings.no_build_isolation_package.is_empty() {
	BuildIsolation::Isolated
	} else {
	environment = PythonEnvironment::from_interpreter(target.interpreter().clone());
	BuildIsolation::SharedPackage(&environment, &settings.no_build_isolation_package)
	};
	// Resolve the flat indexes from `--find-links`.
	let flat_index = {
	let client = FlatIndexClient::new(&client, cache);
	let entries = client
	.fetch(settings.index_locations.flat_indexes().map(Index::url))
	.await?;
	FlatIndex::from_entries(entries, None, &hasher, &settings.build_options)
	};
	// Create a build dispatch.
	let build_dispatch = BuildDispatch::new(
	&client,
	cache,
	build_constraints,
	target.interpreter(),
	&settings.index_locations,
	&flat_index,
	&settings.dependency_metadata,
	state.clone(),
	settings.index_strategy,
	&settings.config_setting,
	build_isolation,
	settings.link_mode,
	&settings.build_options,
	&build_hasher,
	settings.exclude_newer,
	LowerBound::default(),
	sources,
	concurrency,
	preview,
	);
	requirements.extend(
	NamedRequirementsResolver::new(
	&hasher,
	state.index(),
	DistributionDatabase::new(&client, &build_dispatch, concurrency.downloads),
	)
	.with_reporter(Arc::new(ResolverReporter::from(printer)))
	.resolve(unnamed.into_iter())
	.await?,
	);
	}
	requirements
	};

.