fix GIT_TRACE segfault with shell-quoted aliases

The alias argv comes from the split_cmdline function, which
splits the config text for the alias into an array of
strings. It returns the number of elements in the array, but
does not actually put a NULL at the end of the array.
Later, the trace function tries to print this argv and
assumes that it has the trailing NULL.

The split_cmdline function is probably at fault, since argv
lists almost always end with a NULL signal. This patch adds
one, in addition to the returned count; this doesn't hurt
the other callers at all, since they were presumably using
the count already (and will never look at the NULL).

While we're there and using ALLOC_GROW, let's clean up the
other manual grow.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

alias.c

@@ -38,10 +38,7 @@ int split_cmdline(char *cmdline, const char ***argv)
 			while (cmdline[++src]
 					&& isspace(cmdline[src]))
 				; /* skip */
-			if (count >= size) {
-				size += 16;
-				*argv = xrealloc(*argv, sizeof(char*) * size);
-			}
+			ALLOC_GROW(*argv, count+1, size);
 			(*argv)[count++] = cmdline + dst;
 		} else if (!quoted && (c == '\'' || c == '"')) {
 			quoted = c;
@@ -72,6 +69,9 @@ int split_cmdline(char *cmdline, const char ***argv)
 		return error("unclosed quote");
 	}
 
+	ALLOC_GROW(*argv, count+1, size);
+	(*argv)[count] = NULL;
+
 	return count;
 }