sparse-checkout: create builtin with 'list' subcommand

The sparse-checkout feature is mostly hidden to users, as its
only documentation is supplementary information in the docs for
'git read-tree'. In addition, users need to know how to edit the
.git/info/sparse-checkout file with the right patterns, then run
the appropriate 'git read-tree -mu HEAD' command. Keeping the
working directory in sync with the sparse-checkout file requires
care.

Begin an effort to make the sparse-checkout feature a porcelain
feature by creating a new 'git sparse-checkout' builtin. This
builtin will be the preferred mechanism for manipulating the
sparse-checkout file and syncing the working directory.

The documentation provided is adapted from the "git read-tree"
documentation with a few edits for clarity in the new context.
Extra sections are added to hint toward a future change to
a more restricted pattern set.

Helped-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 89 insertions, 0 deletions

diff --git a/Documentation/git-sparse-checkout.txt b/Documentation/git-sparse-checkout.txt
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+git-sparse-checkout(1)
+======================
+
+NAME
+----
+git-sparse-checkout - Initialize and modify the sparse-checkout
+configuration, which reduces the checkout to a set of paths
+given by a list of atterns.
+
+
+SYNOPSIS
+--------
+[verse]
+'git sparse-checkout <subcommand> [options]'
+
+
+DESCRIPTION
+-----------
+
+Initialize and modify the sparse-checkout configuration, which reduces
+the checkout to a set of paths given by a list of patterns.
+
+THIS COMMAND IS EXPERIMENTAL. ITS BEHAVIOR, AND THE BEHAVIOR OF OTHER
+COMMANDS IN THE PRESENCE OF SPARSE-CHECKOUTS, WILL LIKELY CHANGE IN
+THE FUTURE.
+
+
+COMMANDS
+--------
+'list'::
+	Provide a list of the contents in the sparse-checkout file.
+
+
+SPARSE CHECKOUT
+---------------
+
+"Sparse checkout" allows populating the working directory sparsely.
+It uses the skip-worktree bit (see linkgit:git-update-index[1]) to tell
+Git whether a file in the working directory is worth looking at. If
+the skip-worktree bit is set, then the file is ignored in the working
+directory. Git will not populate the contents of those files, which
+makes a sparse checkout helpful when working in a repository with many
+files, but only a few are important to the current user.
+
+The `$GIT_DIR/info/sparse-checkout` file is used to define the
+skip-worktree reference bitmap. When Git updates the working
+directory, it updates the skip-worktree bits in the index based
+on this file. The files matching the patterns in the file will
+appear in the working directory, and the rest will not.
+
+## FULL PATTERN SET
+
+By default, the sparse-checkout file uses the same syntax as `.gitignore`
+files.
+
+While `$GIT_DIR/info/sparse-checkout` is usually used to specify what
+files are included, you can also specify what files are _not_ included,
+using negative patterns. For example, to remove the file `unwanted`:
+
+----------------
+/*
+!unwanted
+----------------
+
+Another tricky thing is fully repopulating the working directory when you
+no longer want sparse checkout. You cannot just disable "sparse
+checkout" because skip-worktree bits are still in the index and your working
+directory is still sparsely populated. You should re-populate the working
+directory with the `$GIT_DIR/info/sparse-checkout` file content as
+follows:
+
+----------------
+/*
+----------------
+
+Then you can disable sparse checkout. Sparse checkout support in 'git
+checkout' and similar commands is disabled by default. You need to
+set `core.sparseCheckout` to `true` in order to have sparse checkout
+support.
+
+SEE ALSO
+--------
+
+linkgit:git-read-tree[1]
+linkgit:gitignore[5]
+
+GIT
+---
+Part of the linkgit:git[1] suite