Depending on whether the index map is case-sensitive or insensitive, we
need to call either `git_idxmap_icase_resize` or `git_idxmap_resize`.
There are multiple locations where we thus use the following pattern:

	if (index->ignore_case &&
	    git_idxmap_icase_resize(map, length) < 0)
		return -1;
	else if (git_idxmap_resize(map, length) < 0)
		return -1;

The funny thing is: on case-insensitive systems, we will try to resize
the map twice in case where `git_idxmap_icase_resize()` doesn't error.
While this will still use the correct hashing function as both map types
use the same, this bug will at least cause us to resize the map twice in
a row.

Fix the issue by introducing a new function `index_map_resize` that
handles case-sensitivity, similar to how `index_map_set` and
`index_map_delete`. Convert all call sites where we were previously
resizing the map to use that new function.