git-gui: Reshow diff if we sent the file to update-index.

git-gui: Reshow diff if we sent the file to update-index.

We can't ask the diff viewer to recompute the diff until after our
update-index child process terminates, as the diff programs need to
be able to read the updated index in order to generate the correct
diff.  This is actually why we prevent diffs from being generated
while there is an update lock on the index, which is why we ignored
our own show_diff invocation in the middle of the write_update_index
event handler.

So now we mark a flag if we identify that the file currently in the
diff viewer was also sent to update-index; then later when the
update-index process has terminated we update the diff viewer if
the flag is true.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>