prepare_{git,shell}_cmd: use argv_array

These functions transform an existing argv into one suitable
for exec-ing or spawning via git or a shell. We can use an
argv_array in each to avoid dealing with manual counting and
allocation.

This also makes the memory allocation more clear and fixes
some leaks. In prepare_shell_cmd, we would sometimes
allocate a new string with "$@" in it and sometimes not,
meaning the caller could not correctly free it. On the
non-Windows side, we are in a child process which will
exec() or exit() immediately, so the leak isn't a big deal.
On Windows, though, we use spawn() from the parent process,
and leak a string for each shell command we run. On top of
that, the Windows code did not free the allocated argv array
at all (but does for the prepare_git_cmd case!).

By switching both of these functions to write into an
argv_array, we can consistently free the result as
appropriate.

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

exec_cmd.c

@@ -1,6 +1,7 @@
 #include "cache.h"
 #include "exec_cmd.h"
 #include "quote.h"
+#include "argv-array.h"
 #define MAX_ARGS	32
 
 static const char *argv_exec_path;
@@ -107,32 +108,25 @@ void setup_path(void)
 	strbuf_release(&new_path);
 }
 
-const char **prepare_git_cmd(const char **argv)
+const char **prepare_git_cmd(struct argv_array *out, const char **argv)
 {
-	int argc;
-	const char **nargv;
-
-	for (argc = 0; argv[argc]; argc++)
-		; /* just counting */
-	nargv = xmalloc(sizeof(*nargv) * (argc + 2));
-
-	nargv[0] = "git";
-	for (argc = 0; argv[argc]; argc++)
-		nargv[argc + 1] = argv[argc];
-	nargv[argc + 1] = NULL;
-	return nargv;
+	argv_array_push(out, "git");
+	argv_array_pushv(out, argv);
+	return out->argv;
 }
 
 int execv_git_cmd(const char **argv) {
-	const char **nargv = prepare_git_cmd(argv);
-	trace_argv_printf(nargv, "trace: exec:");
+	struct argv_array nargv = ARGV_ARRAY_INIT;
+
+	prepare_git_cmd(&nargv, argv);
+	trace_argv_printf(nargv.argv, "trace: exec:");
 
 	/* execvp() can only ever return if it fails */
-	sane_execvp("git", (char **)nargv);
+	sane_execvp("git", (char **)nargv.argv);
 
 	trace_printf("trace: exec failed: %s\n", strerror(errno));
 
-	free(nargv);
+	argv_array_clear(&nargv);
 	return -1;
 }