git-gui: Correctly handle files containing LF in their name.

git-gui: Correctly handle files containing LF in their name.

If we are given a file whose path name contains an LF (\n) we now
escape it by inserting the common escape string \n instead of the
LF character whenever we display the name in the UI.  This way the
text fields don't start to span multiple lines just to display one
file, and it keeps the line numbers correct within the file lists.

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