git-gui: Refresh file status description after hunk application

git-gui: Refresh file status description after hunk application

If we apply a hunk in either direction this may change the file's
status.  For example if a file is completely unstaged, and has at
least two hunks in it and the user stages one hunk the file will
change from "Modified, not staged" to "Portions staged for commit".

Resetting the file path causes our trace on this variable to fire;
that trace is used to update the file header in the diff viewer to
the file's current status.

Noticed by Johannes Sixt.

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