git-gui: Don't use '$$cr master' with aspell earlier than 0.60

git-gui: Don't use '$$cr master' with aspell earlier than 0.60

Apparently aspell 0.50 does not recognize "$$cr master" as a command,
but instead tries to offer suggestions for how to correctly spell
the word "cr".  This is not quite what we are after when we want
the name of the current dictionary.

Instead of locking up git-gui waiting for a response that may never
come back from aspell we avoid sending this command if the binary
we have started claims to be before version 0.60.

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