convert: avoid malloc of original file size

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>