[PATCH v3 2/6] string-list: introduce `string_list_setlen()`

It is sometimes useful to reduce the size of a `string_list`'s list of items without having to re-allocate them. For example, doing the following:

    struct strbuf buf = STRBUF_INIT;
    struct string_list parts = STRING_LIST_INIT_NO_DUP;
    while (strbuf_getline(&buf, stdin) != EOF) {
      parts.nr = 0;
      string_list_split_in_place(&parts, buf.buf, ":", -1);
      /* ... */
    }
    string_list_clear(&parts, 0);

is preferable over calling `string_list_clear()` on every iteration of the loop. This is because `string_list_clear()` causes us free our existing `items` array. This means that every time we call `string_list_split_in_place()`, the string-list internals re-allocate the same size array.

Since in the above example we do not care about the individual parts after processing each line, it is much more efficient to pretend that there aren't any elements in the `string_list` by setting `list->nr` to 0 while leaving the list of elements allocated as-is.

This allows `string_list_split_in_place()` to overwrite any existing entries without needing to free and re-allocate them.

However, setting `list->nr` manually is not safe in all instances. There are a couple of cases worth worrying about:

  - If the `string_list` is initialized with `strdup_strings`,
    truncating the list can lead to overwriting strings which are
    allocated elsewhere. If there aren't any other pointers to those
    strings other than the ones inside of the `items` array, they will
    become unreachable and leak.

    (We could ourselves free the truncated items between
    string_list->items[nr] and `list->nr`, but no present or future
    callers would benefit from this additional complexity).

  - If the given `nr` is larger than the current value of `list->nr`,
    we'll trick the `string_list` into a state where it thinks there are
    more items allocated than there actually are, which can lead to
    undefined behavior if we try to read or write those entries.

Guard against both of these by introducing a helper function which guards assignment of `list->nr` against each of the above conditions.

Co-authored-by: Jeff King <peff@peff.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 string-list.c |  9 +++++++++
 string-list.h | 10 ++++++++++
 2 files changed, 19 insertions(+)