git: refactor alias handling to use a `struct strvec`

git: refactor alias handling to use a `struct strvec`

In `handle_alias()` we use both `argcp` and `argv` as in-out parameters.
Callers mostly pass through the static array from `main()`, but once we
handle an alias we replace it with an allocated array that may contain
some allocated strings. Callers do not handle this scenario at all and
thus leak memory.

We could in theory handle the lifetime of `argv` in a hacky fashion by
letting callers free it in case they see that an alias was handled. But
while that would likely work, we still wouldn't be able to easily handle
the lifetime of strings referenced by `argv`.

Refactor the code to instead use a `struct strvec`, which effectively
removes the need for us to manually track lifetimes.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>