Sparse checkout inconsistency with non-folder paths between cone mode and full matching (2.5.0)

[Finn Bryant <finnbryant@gmail.com>]

Hi,

With cone mode enabled for a sparse checkout, a pattern like the
following is accepted:

/*
!/*/
/a_file_or_folder/

As the name suggests, a_file_or_folder might be a file, or might be a
directory. If it's a directory, then this is a "valid" recursive match
for the directory and everything works as expected.
But if the path leads to an ordinary file, it appears that cone mode
will *exclude* the file from the worktree (ie, it'll have the
skip-worktree flag set).
This seems bizarre and unexpected behaviour.

My suspicion is that cone mode is supposed to be a pure subset of full
pattern matching, such that if cone mode is ever disabled, the
sparseness of the worktree will be unchanged. Clearly, this scenario
is breaking that pattern.

I think the correct behaviour is that recursive matches for a
potential directory do not have any effect on a non-directory file
with the same name. Alternatively, you could forbid any patterns which
match non-directory files instead (and downgrade to full pattern
matching), though I'd not be a fan of this, since it'd mean the
validity of a cone-mode sparse configuration file is dependent on the
contents of the repo, and is thus much harder to ascertain (scripts
can't simply prove if it's a valid cone mode file by parsing it,
they'd need to have access to any repo it may be applied to, and
inspect the type of any matching file/folder paths, and its validity
could be changed simply by replacing a folder with a file in the
repo).

If matching behaviour with full pattern mode is a non-goal for cone
mode, I'd still question the logic of this behaviour, though I suppose
it does have the benefit of (accidentally?) adding support for
excluding individual files from a sparse checkout, which I imagine
some could find useful. Personally I'd prefer that was instead added
with a more sane syntax, if needed.

Full test case:

$ git init test_repo
Initialized empty Git repository in [path]/test_repo/.git/
$ cd test_repo/
$ touch some_file a_file_or_folder
$ git add some_file a_file_or_folder
$ git commit -m "some files"
[master (root-commit) 80d5918] some files
 2 files changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 a_file_or_folder
 create mode 100644 some_file
$ git sparse-checkout init --cone
$ git read-tree -mu HEAD
$ ls -1
a_file_or_folder
some_file
$ git sparse-checkout set a_file_or_folder
$ git read-tree -mu HEAD
$ ls -1
some_file
$ cat .git/info/sparse-checkout
/*
!/*/
/a_file_or_folder/
$


Right now I'm trying to integrate cone mode with my company's existing
manifest infrastructure, which doesn't differentiate between files and
folders, so this is forcing me to perform a lot of repo checks to
confirm we aren't accidentally excluding files we were supposed to
include. Just in case you needed another example of why this behaviour
is unhelpful.