csum-file: introduce discard_hashfile()

csum-file: introduce discard_hashfile()

The hashfile API is used to write out a "hashfile", which has a
final checksum (typically SHA-1) at the end.  An in-core hashfile
structure has up to two file descriptors and a few buffers that can
only be freed by calling a helper function that is private to the
csum-file implementation.

The usual flow of a user of the API is to first open a file
descriptor for writing, obtain a hashfile associated with that write
file descriptor by calling either hashfd() or hashfd_check(), call
hashwrite() number of times to write data to the file, and then call
finalize_hashfile(), which appends th checksum to the end of the
file, closes file descriptors and releases associated buffers.

But what if a caller finds some error after calling hashfd() to
start the process and/or hashwrite() to send some data to the file,
and wants to abort the operation?  The underlying file descriptor is
often managed by the tempfile API, so aborting will clean the file
out of the filesystem, but the resources associated with the in-core
hashfile structure is lost.

Introduce discard_hashfile() API function to allow them to release
the resources held by a hashfile structure the callers want to
dispose of, and use that in read-cache.c:do_write_index(), which is
a central place that writes the index file.

Mark t2107 as leak-free, as this leak in "update-index --cacheinfo"
test that deliberately makes it fail is now plugged.

Signed-off-by: Junio C Hamano <gitster@pobox.com>