convert: avoid malloc of original file size

We write the output of a "clean" filter into a strbuf. Rather than growing the strbuf dynamically as we read its output, we make the initial allocation as large as the original input file. This is a good guess when the filter is just tweaking a few bytes, but it's disastrous when the point of the filter is to condense a very large file into a short identifier (e.g., the way git-lfs and git-annex do). We may ask to allocate many gigabytes, causing the allocation to fail and Git to die(). Instead, let's just let strbuf do its usual growth. When the clean filter does output something around the same size as the worktree file, the buffer will need to be reallocated until it fits, starting at 8192 and doubling in size. Benchmarking indicates that reallocation is not a significant overhead for outputs up to a few MB in size. Signed-off-by: Joey Hess <id@joeyh.name> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>