Define our own extended CmdLineParser for extra parsing support

Define our own extended CmdLineParser for extra parsing support

A specialized CmdLineParser subclass is used to automatically handle
`--git-dir=foo`, as args4j wants to see `--git-dir foo` instead.
We split on the first equal sign on a long option and use that to
delimit the value from the option name.

We stop this long option fixup at the first -- found on the command
line, as this is a traditional delimiter between options and only
arguments.  This is an assumption that all of our users of a command
line parser support would want to terminate option parsing at the
first -- they see on the command line, and thus it is OK to stop
our GNU long optino style cleanup.

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