v2git-submodule.sh: shorten submodule SHA-1s using rev-parse

> Until now, `git submodule summary` was always emitting 7-character
> SHA-1s that have a higher chance of being ambiguous for larger
> repositories.  Use `git rev-parse --short` instead, which will
> determine suitable short SHA-1 lengths.
>
> We cannot rely on always successfully invoking `git rev-parse` in the
> submodule directory.  Keep the old method using `cut` as a fallback.
>
> Signed-off-by: Sven van Haastregt <svenvh@gmail.com>
> ---
> diff --git a/git-submodule.sh b/git-submodule.sh
> @@ -850,8 +850,16 @@ cmd_summary() {
> +               # Ensure we have crudely abbreviated SHA-1s as fallback in case
> +               # rev-parse fails to shorten the SHA-1s.
>                 sha1_abbr_src=$(echo $sha1_src | cut -c1-7)
>                 sha1_abbr_dst=$(echo $sha1_dst | cut -c1-7)
> +
> +               sha1_abbrev=$(GIT_DIR="$name/.git" git rev-parse --short $sha1_src) &&
> +               sha1_abbr_src=$sha1_abbrev
> +               sha1_abbrev=$(GIT_DIR="$name/.git" git rev-parse --short $sha1_dst) &&
> +               sha1_abbr_dst=$sha1_abbrev

> This could be made a bit easier to follow by using indentation and ||
> rather than &&. For instance, rewriting the entire block as:
>
>     # Shorten with hard-coded fallback if rev-parse fails
>     sha1_abbr_src=$(GIT_DIR="$name/.git" git rev-parse --short $sha1_src ||
>         echo $sha1_src | cut -c1-7)
>     sha1_abbr_dst=$(GIT_DIR="$name/.git" git rev-parse --short $sha1_dst ||
>         echo $sha1_dst | cut -c1-7)
>
> In fact, the code is clear enough that the comment isn't even needed.
>
> By the way, if git-rev-parse does fail, is it going to produce an
> error message on stderr that needs to be suppressed?