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.h

@@ -1,11 +1,13 @@
 #ifndef GIT_EXEC_CMD_H
 #define GIT_EXEC_CMD_H
 
+struct argv_array;
+
 extern void git_set_argv_exec_path(const char *exec_path);
 extern const char *git_extract_argv0_path(const char *path);
 extern const char *git_exec_path(void);
 extern void setup_path(void);
-extern const char **prepare_git_cmd(const char **argv);
+extern const char **prepare_git_cmd(struct argv_array *out, const char **argv);
 extern int execv_git_cmd(const char **argv); /* NULL terminated */
 LAST_ARG_MUST_BE_NULL
 extern int execl_git_cmd(const char *cmd, ...);