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: Shawn O. Pearce <spearce@spearce.org>

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