completion: clean up the __git_find_on_cmdline() helper function

completion: clean up the __git_find_on_cmdline() helper function

The __git_find_on_cmdline() helper function started its life as
__git_find_subcommand() [1], but it served a more general purpose than
looking for subcommands, so later it was renamed accordingly [2].
However, that rename didn't touch the body of the function, and left
the $subcommand local variable behind, still reminiscent of the
function's original purpose.

Let's clean up the names of __git_find_on_cmdline()'s local variables
and get rid of that $subcommand variable name.

While at it, add a short comment describing the function's purpose.

[1] 3ff1320d4b (bash: refactor searching for subcommands on the
    command line, 2008-03-10),
[2] 918c03c2a7 (bash: rename __git_find_subcommand() to
    __git_find_on_cmdline(), 2009-09-15)

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>