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>

contrib/completion/git-completion.bash

@@ -1070,14 +1070,17 @@ __git_aliased_command ()
 }
 
 # __git_find_on_cmdline requires 1 argument
+# Check whether one of the given words is present on the command line,
+# and print the first word found.
 __git_find_on_cmdline ()
 {
-	local word subcommand c=1
+	local word c=1
+	local wordlist="$1"
+
 	while [ $c -lt $cword ]; do
-		word="${words[c]}"
-		for subcommand in $1; do
-			if [ "$subcommand" = "$word" ]; then
-				echo "$subcommand"
+		for word in $wordlist; do
+			if [ "$word" = "${words[c]}" ]; then
+				echo "$word"
 				return
 			fi
 		done