git-gui: Smarter command line parsing for browser, blame

git-gui: Smarter command line parsing for browser, blame

The browser subcommand now optionally accepts a single revision
argument; if no revision argument is supplied then we use the
current branch as the tree to browse.  This is very common, so
its a nice option.

Our blame subcommand now tries to perform the same assumptions
as the command line git-blame; both the revision and the file
are optional.  We assume the argument is a filename if the file
exists in the working directory, otherwise we assume the argument
is a revision name.  A -- can be supplied between the two to force
parsing, or before the filename to force it to be a filename.

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