bash: rename __git_find_subcommand() to __git_find_on_cmdline()

bash: rename __git_find_subcommand() to __git_find_on_cmdline()

__git_find_subcommand() was originally meant to check whether
subcommands are already present on the command line.  But the code is
general enough to be used for checking the presence of command line
options as well, and the next commit will use it for that purpose, so
let's give it a more general name.

Signed-off-by: SZEDER Gábor <redacted>
Acked-by: Shawn O. Pearce <redacted>
Signed-off-by: Junio C Hamano <redacted>