reset.c: move lock, write and commit out of update_index_refresh()

reset.c: move lock, write and commit out of update_index_refresh()

In preparation for the/a following patch, move the locking, writing
and committing of the index file out of update_index_refresh(). The
code duplication caused will soon be taken care of. What remains of
update_index_refresh() is just one line, but it is still called from
two places, so let's leave it for now.

In the process, we expose and fix the minor UI bug that makes us print
"Could not refresh index" when we fail to write the index file when
invoked with a pathspec. Copy the error message from the pathspec-less
codepath ("Could not write new index file.").

Signed-off-by: Martin von Zweigbergk <martinvonz@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>