git-gui: Move commit_prehook into commit_tree.

git-gui: Move commit_prehook into commit_tree.

The only reason the commit_prehook logic was broken out into its own
proc was so it could be invoked after the current set of files that
were already added to the commit could be refreshed if 'Allow Partially
Added Files' was set to false.  Now that we no longer even offer that
option to the user there is no reason to keep this code broken out
into its own procedure.

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