bash: refactor searching for subcommands on the command line

bash: refactor searching for subcommands on the command line

This patch adds the __git_find_subcommand function, which takes one
argument: a string containing all subcommands separated by spaces.  The
function searches through the command line whether a subcommand is
already present.  The first found subcommand will be printed to standard
output.

This enables us to remove code duplications from completion functions
for commands having subcommands.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>