git-gui: Always use lsearch -exact, to prevent globbing.

git-gui: Always use lsearch -exact, to prevent globbing.

Anytime we are using lsearch we are doing [lsearch -sorted] and we
are applying it to file paths (or file path like things).  Its valid
for these to contain special glob characters, but when that happens
we do not want globbing to occur.  Instead we really need exact
match semantics.  Always supplying -exact to lsearch will ensure that
is the case.

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