git-gui: Correctly report failures from git-write-tree

If git-write-tree fails (such as if the index file is currently
locked and it wants to write to it) we were not getting the error
message as $tree_id was always the empty string so we shortcut
through the catch and never got the output from stderr.

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

lib/commit.tcl

@@ -253,7 +253,7 @@ proc commit_committree {fd_wt curHEAD msg} {
 	global repo_config
 
 	gets $fd_wt tree_id
-	if {$tree_id eq {} || [catch {close $fd_wt} err]} {
+	if {[catch {close $fd_wt} err]} {
 		error_popup "write-tree failed:\n\n$err"
 		ui_status {Commit failed.}
 		unlock_index