Git lets users define the default update/checkout strategy for a submodule
by setting the `submodule.<name>.update` key in `.gitmodules` file.

If the update strategy is `none`, the submodule will be skipped during
update. It will not be fetched and checked out:

1. *foo* is a big git repo

```
/tmp $ git init foo
Initialized empty Git repository in /tmp/foo/.git/
/tmp $ dd if=/dev/zero of=foo/big bs=1000M count=1
1+0 records in
1+0 records out
1048576000 bytes (1.0 GB, 1000 MiB) copied, 0.482087 s, 2.2 GB/s
/tmp $ git -C foo add big
/tmp $ git -C foo commit -m 'I am big'
[main (root-commit) 84fb533] I am big
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 big
```

2. *bar* is a repo with a big submodule with `update=none`

```
/tmp $ git init bar
Initialized empty Git repository in /tmp/bar/.git/
/tmp $ git -C bar submodule add file:///tmp/foo foo
Cloning into '/tmp/bar/foo'...
remote: Enumerating objects: 3, done.
remote: Counting objects: 100% (3/3), done.
remote: Total 3 (delta 0), reused 1 (delta 0), pack-reused 0
Receiving objects: 100% (3/3), 995.50 KiB | 338.00 KiB/s, done.
/tmp $ git -C bar config --file .gitmodules submodule.foo.update none
/tmp $ cat bar/.gitmodules
[submodule "foo"]
        path = foo
        url = file:///tmp/foo
        update = none
/tmp $ git -C bar commit --all -m 'I have a big submodule with update=none'
[main (root-commit) 6c355ea] I have a big submodule not updated by default
 2 files changed, 4 insertions(+)
 create mode 100644 .gitmodules
 create mode 160000 foo
```

3. *baz* is a clone of *bar*, notice *foo* submodule gets skipped

```
/tmp $ git clone --recurse-submodules file:///tmp/bar baz
Cloning into 'baz'...
remote: Enumerating objects: 3, done.
remote: Counting objects: 100% (3/3), done.
remote: Compressing objects: 100% (3/3), done.
remote: Total 3 (delta 0), reused 0 (delta 0), pack-reused 0
Receiving objects: 100% (3/3), done.
Submodule 'foo' (file:///tmp/foo) registered for path 'foo'
Skipping submodule 'foo'
/tmp $ git -C baz submodule update --init
Skipping submodule 'foo'
/tmp $
```

Cargo, on the other hand, ignores the submodule update strategy set in
`.gitmodules` properties when updating dependencies. Such behavior can
be considered against the wish of the crate publisher.

4. *bar* is now a lib with a big submodule with update disabled

```
/tmp $ cargo init --lib bar
     Created library package
/tmp $ git -C bar add .
/tmp $ git -C bar commit -m 'I am a lib with a big submodule but update=none'
[main eb07cf7] I am a lib with a big submodule but update=none
 3 files changed, 18 insertions(+)
 create mode 100644 .gitignore
 create mode 100644 Cargo.toml
 create mode 100644 src/lib.rs
/tmp $
```

5. *qux* depends on *bar*, notice *bar*'s submodules are fetched

```
/tmp $ cargo init qux && cd qux
     Created binary (application) package
/tmp/qux $ echo -e '[dependencies.bar]\ngit = "file:///tmp/bar"' >> Cargo.toml
/tmp/qux $ time cargo update
    Updating git repository `file:///tmp/bar`
    Updating git submodule `file:///tmp/foo`

real    0m22.182s
user    0m20.402s
sys     0m1.714s
/tmp/qux $
```

Fix it by checking if a Git repository submodule should be updated when
cargo processes dependencies.

6. With the change applied, submodules with `update=none` are skipped

```
/tmp/qux $ cargo cache -a > /dev/null
/tmp/qux $ time ~/src/cargo/target/debug/cargo update
    Updating git repository `file:///tmp/bar`
    Skipping git submodule `file:///tmp/foo`

real    0m0.029s
user    0m0.021s
sys     0m0.008s
/tmp/qux $
```

Fixes rust-lang#4247.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>