git-gui: Change our initial GC hint to be an estimate

Instead of running a full git-count-objects to count all of the loose
objects we can get a reasonably close approximation by counting the
number of files in the .git/objects/42 subdirectory. This works out
reasonably well because the SHA-1 hash has a fairly even distribution,
so every .git/objects/?? subdirectory should get a relatively equal
number of files.  If we have at least 8 files in .git/objects/42 than it
is very likely there is about 8 files in every other directory, leaving
us with around 2048 loose objects.

This check is much faster, as we need to only perform a readdir of
a single directory, and we can do it directly from Tcl and avoid the
costly fork+exec.

All of the credit on how clever this is goes to Linus Torvalds; he
suggested using this trick in a post commit hook to repack every so
often.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -2560,12 +2560,21 @@ if {[is_enabled transport]} {
 # -- Only suggest a gc run if we are going to stay running.
 #
 if {[is_enabled multicommit]} {
-	set object_limit 2000
-	if {[is_Windows]} {set object_limit 200}
-	regexp {^([0-9]+) objects,} [git count-objects] _junk objects_current
+	set object_limit 8
+	if {[is_Windows]} {
+		set object_limit 1
+	}
+	set objects_current [llength [glob \
+		-directory [gitdir objects 42] \
+		-nocomplain \
+		-tails \
+		-- \
+		*]]
 	if {$objects_current >= $object_limit} {
+		set objects_current [expr {$objects_current * 256}]
+		set object_limit    [expr {$object_limit    * 256}]
 		if {[ask_popup \
-			"This repository currently has $objects_current loose objects.
+			"This repository currently has approximately $objects_current loose objects.
 
 To maintain optimal performance it is strongly recommended that you compress the database when more than $object_limit loose objects exist.
 
@@ -2573,7 +2582,7 @@ Compress the database now?"] eq yes} {
 			do_gc
 		}
 	}
-	unset object_limit _junk objects_current
+	unset object_limit objects_current
 }
 
 lock_index begin-read