[PATCH 10/10] .sparse: add in-tree sparse-checkout for Git

["Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>]

From: Derrick Stolee <dstolee@microsoft.com>

The in-tree sparse-checkout feature allows architects to adapt their
dependentent subsystems into sparse-checkout specificiations. This helps
the typical users who do not know the full build system to use the
sparse-chekcout in a pain-free way. In particular, if the
sparse-checkout dependencies update at the same times as build
dependencies, then the users automatically get new sparse-checkout
definitions as they switch branches.

For the Git codebase, it is not immediately obvious which directories
are absolutely required for building Git. From my estimation, the
necessary directories for building and testing Git on Linux are listed
in the .sparse/base.deps file in this change. A few more directories in
the compat/ dir are required for building on Windows.

This presents a new possible workflow for Git contributors, especially
those that want to test several new features in their workflow. The
following allows a user to set up working on Git with partial clone and
sparse-checkout:

 $ git clone --sparse --filter=blob:none https://github.com/git/git
 $ cd git
 $ git sparse-checkout set --in-tree .sparse/base.deps
 $ make test

Perhaps there are ways we can further reduce the size of the "bare
necessities" by rearranging code and adjusting the Makefile to match. In
particular, I noticed that the vcs-svn directory is required for the
build. It could be helpful to create a way to build a very small version
of "core Git" that doesn't include integrations with other version
control systems, and having them automatically disabled if they are
missing from the sparse-checkout definition would be a great way to make
that more accessible for contributors.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
 .sparse/base.deps    | 19 +++++++++++++++++++
 .sparse/windows.deps |  3 +++
 2 files changed, 22 insertions(+)
 create mode 100644 .sparse/base.deps
 create mode 100644 .sparse/windows.deps

diff --git a/.sparse/base.deps b/.sparse/base.deps
new file mode 100644
index 00000000000..b0682175dc3
--- /dev/null
+++ b/.sparse/base.deps
@@ -0,0 +1,19 @@
+[sparse]
+	dir = Documentation
+	dir = block-sha1
+	dir = builtin
+	dir = compat/.depend
+	dir = contrib
+	dir = ewah
+	dir = gitweb
+	dir = mergetools
+	dir = negotiator
+	dir = perl
+	dir = refs
+	dir = sha1dc
+	dir = sha256
+	dir = t
+	dir = templates
+	dir = trace2
+	dir = vcs-svn
+	dir = xdiff
diff --git a/.sparse/windows.deps b/.sparse/windows.deps
new file mode 100644
index 00000000000..6c9bf1df335
--- /dev/null
+++ b/.sparse/windows.deps
@@ -0,0 +1,3 @@
+[sparse]
+	inherit = .sparse/base.deps
+	dir = compat
-- 
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