log-tree: fix patch filename computation in "git format-patch"

log-tree: fix patch filename computation in "git format-patch"

When using "git format-patch", "get_patch_filename" in
"log-tree.c" calls "strbuf_splice" that could die with
the following message:

"`pos + len' is too far after the end of the buffer"

if you have:

	buf->len < start_len + FORMAT_PATCH_NAME_MAX

but:

	buf->len + suffix_len > start_len + FORMAT_PATCH_NAME_MAX

This patch tries to get rid of that bug.

[jc: w/ simplified logic]

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>