git-gui: Use builtin version of 'git gc'.

Technically the new git-gc command is strictly Porcelain; its invoking
multiple plumbing commands to do its work.  Since git-gui tries to not
rely on Porclain we shouldn't be invoking git-gc directly, instead we
should perform its tasks on our own.

To make this easy I've created console_chain, which takes a list of
tasks to perform and runs them all in the same console window.  If
any individual task fails then the chain stops running and the window
shows a failure bar.  Only once all tasks have been completed will it
invoke console_done with a successful status.

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

git-gui.sh

@@ -2917,9 +2917,45 @@ proc console_read {w fd after} {
 	fconfigure $fd -blocking 0
 }
 
-proc console_done {w ok} {
+proc console_chain {cmdlist w {ok 1}} {
+	if {$ok} {
+		if {[llength $cmdlist] == 0} {
+			console_done $w $ok
+			return
+		}
+
+		set cmd [lindex $cmdlist 0]
+		set cmdlist [lrange $cmdlist 1 end]
+
+		if {[lindex $cmd 0] eq {console_exec}} {
+			console_exec $w \
+				[lindex $cmd 1] \
+				[list console_chain $cmdlist]
+		} else {
+			uplevel #0 $cmd $cmdlist $w $ok
+		}
+	} else {
+		console_done $w $ok
+	}
+}
+
+proc console_done {args} {
 	global console_cr console_data
 
+	switch -- [llength $args] {
+	2 {
+		set w [lindex $args 0]
+		set ok [lindex $args 1]
+	}
+	3 {
+		set w [lindex $args 1]
+		set ok [lindex $args 2]
+	}
+	default {
+		error "wrong number of args: console_done ?ignored? w ok"
+	}
+	}
+
 	if {$ok} {
 		if {[winfo exists $w]} {
 			$w.m.s conf -background green -text {Success}
@@ -3038,7 +3074,12 @@ proc do_stats {} {
 
 proc do_gc {} {
 	set w [new_console {gc} {Compressing the object database}]
-	console_exec $w {git gc} console_done
+	console_chain {
+		{console_exec {git pack-refs --prune}}
+		{console_exec {git reflog expire --all}}
+		{console_exec {git repack -a -d -l}}
+		{console_exec {git rerere gc}}
+	} $w
 }
 
 proc do_fsck_objects {} {