git-gui: Update in memory states after commit.

git-gui: Update in memory states after commit.

In order to allow the user to toggle include/exclude from next commit
for files which were partially included in the last commit we need the
current index mode+sha1 data stored in our file_states array.  For
any partially included file we have this information from diff-files,
so we just have to copy it over to the diff-index portion of our state
array.

Signed-off-by: Shawn O. Pearce <redacted>