git-gui: Automatically update-index all included files before commit.

git-gui: Automatically update-index all included files before commit.

If the user has "Allow Partially Included Files" disabled (and most
probably will as its the default setting) we should run update-index
on every included file before commit to make sure that any changes
made by the user since the last rescan will still be part of this
commit.

If we don't update-index every modified file the user will likely
become confused when part of their changes were committed and other
parts weren't; and those other parts won't show up until a later
rescan occurs.  Since we don't rescan immediately after a commit
this may be a while.

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