git-gui: Always use eq/ne for string comparsions.

git-gui: Always use eq/ne for string comparsions.

This is one of those stupid Tcl mistakes that an experienced Tcl
programmer just wouldn't make.  We should always use eq and ne to
compare string values (and never == or !=) as when we use ==/!=
Tcl will attempt to convert either side to numeric if one of the
two sides looks like a numeric.  This could cause some trouble if
a file named "1" exists and a different file named "1.0" also exists;
their paths are equal according to == but not according to eq.

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