Sparse checkout completion fixes

This fixes a few issues with tab completion for the sparse-checkout command, specifically with the "add" and "set" subcommands.

Changes since v2:

* For patch 4, provide completion (and startings) of arguments into patterns that match files in the index -- as suggested by Junio.

Changes since v1:

* Use __git wrapper function to squelch errors, as suggested by SZEDER Gábor
* note that we could use the index or HEAD for the more involved solution in patch 4.

[1] https://lore.kernel.org/git/[email protected]/

Elijah Newren (4):
completion: squelch stray errors in sparse-checkout completion
completion: fix logic for determining whether cone mode is active
completion: avoid misleading completions in cone mode
completion: avoid user confusion in non-cone mode

 contrib/completion/git-completion.bash | 123 ++++++++++++++++++++++++-
 t/t9902-completion.sh                  |   9 +-
 2 files changed, 127 insertions(+), 5 deletions(-)

base-commit: 564d0252ca632e0264ed670534a51d18a689ef5d
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1349%2Fnewren%2Fsparse-checkout-completion-fixes-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1349/newren/sparse-checkout-completion-fixes-v3
Pull-Request: https://github.com/gitgitgadget/git/pull/1349

Range-diff vs v2:

 1:  97e20e3b99d = 1:  97e20e3b99d completion: squelch stray errors in sparse-checkout completion
 2:  212ba35ed46 = 2:  212ba35ed46 completion: fix logic for determining whether cone mode is active
 3:  1cbbcd9097c = 3:  1cbbcd9097c completion: avoid misleading completions in cone mode
 4:  604f21dc827 < -:  ----------- completion: avoid user confusion in non-cone mode
 -:  ----------- > 4:  89501b366ff completion: avoid user confusion in non-cone mode

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