string-list: use ALLOC_GROW macro when reallocing string_list

string-list: use ALLOC_GROW macro when reallocing string_list

Use ALLOC_GROW() macro when reallocing a string_list array
rather than simply increasing it by 32.  This is a performance
optimization.

During status on a very large repo and there are many changes,
a significant percentage of the total run time is spent
reallocing the wt_status.changes array.

This change decreases the time in wt_status_collect_changes_worktree()
from 125 seconds to 45 seconds on my very large repository.

This produced a modest gain on my 1M file artificial repo, but
broke even on linux.git.

Test                                            HEAD^^            HEAD
---------------------------------------------------------------------------------------
0005.2: read-tree status br_ballast (1000001)   8.29(5.62+2.62)   8.22(5.57+2.63) -0.8%

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>