git-gui: Use progress bar while resetting/aborting files

Resetting a large number of files on a slow filesystem can take
considerable time, just as switching branches in such a case can
take more than two seconds.  We now take advantage of the progress
meter output by read-tree and show it in the main window status
bar, just like we do during checkout (branch switch).
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

lib/merge.tcl

@@ -233,10 +233,10 @@ Continue with resetting the current changes?"
 	}
 
 	if {[ask_popup $op_question] eq {yes}} {
-		set fd [git_read read-tree --reset -u HEAD]
+		set fd [git_read --stderr read-tree --reset -u -v HEAD]
 		fconfigure $fd -blocking 0 -translation binary
 		fileevent $fd readable [namespace code [list _reset_wait $fd]]
-		ui_status {Aborting... please wait...}
+		$::main_status start {Aborting} {files reset}
 	} else {
 		unlock_index
 	}
@@ -245,9 +245,12 @@ Continue with resetting the current changes?"
 proc _reset_wait {fd} {
 	global ui_comm
 
-	read $fd
+	$::main_status update_meter [read $fd]
+
+	fconfigure $fd -blocking 1
 	if {[eof $fd]} {
-		close $fd
+		set fail [catch {close $fd} err]
+		$::main_status stop
 		unlock_index
 
 		$ui_comm delete 0.0 end
@@ -259,7 +262,12 @@ proc _reset_wait {fd} {
 		catch {file delete [gitdir MERGE_MSG]}
 		catch {file delete [gitdir GITGUI_MSG]}
 
+		if {$fail} {
+			warn_popup "Abort failed.\n\n$err"
+		}
 		rescan {ui_status {Abort completed.  Ready.}}
+	} else {
+		fconfigure $fd -blocking 0
 	}
 }