Remove sparse index expansions from untracked file stashes

[vdye]

Changes

• Reduce the conditions in which the index is expanded in checkout-index --sparse (it's only needed if a sparse directory is present to expand)
• Update unpack_trees to set the result to "sparse" if it does not meet any conditions that require a full index
• Move the stash -u & stash pop test out of its own custom test case and into sparse index is not expanded
  • It no longer expands any index, temporary or not, if all files in the stash are in the checkout cone

Performance

Overall, there weren't substantial differences in the performance of stash. That's more-or-less expected - the index expansions removed only apply to the "temporary" untracked file indexes, which are substantially smaller than the regular repository index.

Test                                                              ms/vfs-2.33.0     remove-index-expansions
-----------------------------------------------------------------------------------------------------------
2000.2: git stash && git stash pop (full-v3)                      4.10(2.77+1.17)   3.92(2.69+1.13) -4.4%  
2000.3: git stash && git stash pop (full-v4)                      3.85(2.61+1.11)   4.19(2.83+1.18) +8.8%  
2000.4: git stash && git stash pop (sparse-v3)                    1.39(0.34+1.87)   1.47(0.35+1.85) +5.8%  
2000.5: git stash && git stash pop (sparse-v4)                    1.44(0.33+1.83)   1.48(0.35+1.78) +2.8%  
2000.6: echo >>new && git stash -u && git stash pop (full-v3)     4.68(3.10+1.54)   4.88(3.19+1.60) +4.3%  
2000.7: echo >>new && git stash -u && git stash pop (full-v4)     4.32(2.95+1.36)   4.24(2.88+1.30) -1.9%  
2000.8: echo >>new && git stash -u && git stash pop (sparse-v3)   1.60(0.39+1.84)   1.77(0.44+2.03) +10.6% 
2000.9: echo >>new && git stash -u && git stash pop (sparse-v4)   1.74(0.43+1.98)   1.76(0.44+2.23) +1.1%

[ldennington]

Great comments and commit description - made it super easy to follow your work!

[derrickstolee]

I see the subtle point here: this isn't really a performance fix, but allows us to use ensure_not_expanded on these two commands (which is a valuable way to protect the performance in the future).