add--interactive: ignore unmerged entries in patch mode

add--interactive: ignore unmerged entries in patch mode

When "add -p" sees an unmerged entry, it shows the combined
diff and then immediately skips the hunk. This can be
confusing in a variety of ways, depending on whether there
are other changes to stage (in which case you get the
superfluous combined diff output in between other hunks) or
not (in which case you get the combined diff and the program
exits immediately, rather than seeing "No changes").

The current behavior was not planned, and is just what the
implementation happens to do. Instead, let's explicitly
remove unmerged entries from our list of modified files, and
print a warning that we are ignoring them.

We can cheaply find which entries are unmerged by adding
"--raw" output to the "diff-files --numstat" we already run.
There is one non-obvious thing we must change when parsing
this combined output. Before this patch, when we saw a
numstat line for a file that did not have index changes, we
would create a new record with 'unchanged' in the 'INDEX'
field.  Because "--raw" comes before "--numstat", we must
move this special-case down to the raw-line case (and it is
sufficient to move it rather than handle it in both places,
since any file which has a --numstat will also have a --raw
entry).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>