git-gui: Don't attempt to inline array reads in methods

git-gui: Don't attempt to inline array reads in methods

If a variable reference to a field is to an array, and it is
the only reference to that field in that method we cannot make
it an inlined [set foo] call as the regexp was converting the
Tcl code wrong.  We were producing "[set foo](x)" for "$foo(x)",
and that isn't valid Tcl when foo is an array.  So we just punt
if the only occurance has a ( after it.

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