[PATCH] sha1_file: pass empty buffer to index empty file

`git add` of an empty file with a filter currently pops complaints from `copy_fd` about a bad file descriptor.

This traces back to these lines in sha1_file.c:index_core:

	if (!size) {
		ret = index_mem(sha1, NULL, size, type, path, flags);

The problem here is that content to be added to the index can be supplied from an fd, or from a memory buffer, or from a pathname. This call is supplying a NULL buffer pointer and a zero size.

Downstream logic takes the complete absence of a buffer to mean the data is to be found elsewhere -- for instance, these, from convert.c:

	if (params->src) {
		write_err = (write_in_full(child_process.in, params->src, params->size) < 0);
	} else {
		write_err = copy_fd(params->fd, child_process.in);
	}

~If there's a buffer, write from that, otherwise the data must be coming from an open fd.~

Perfectly reasonable logic in a routine that's going to write from either a buffer or an fd.

So change `index_core` to supply an empty buffer when indexing an empty file.

There's a patch out there that instead changes the logic quoted above to take a `-1` fd to mean "use the buffer", but it seems to me that the distinction between a missing buffer and an empty one carries intrinsic semantics, where the logic change is adapting the code to handle incorrect arguments.

Signed-off-by: Jim Hill <gjthill@gmail.com>
---
 sha1_file.c                        |  2 +-
 t/t2205-add-empty-filtered-file.sh | 21 +++++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)
 create mode 100755 t/t2205-add-empty-filtered-file.sh