git: avoid segfault on "git --shallow-file" without a value

In "git.c", the other `handle_options()` options that take their value
as a separate argument, like `--git-dir`, `--namespace` or `-C`, check
that such an argument actually exists before using it, and error out
with a message and the usage string otherwise.

The `--shallow-file` option doesn't perform that check. It blindly
advances past the option and then dereferences the next element of
`argv`, which is the NULL terminator when no value was given. So
`git --shallow-file` segfaults:

  $ git --shallow-file
  Segmentation fault (core dumped)

Let's fix that by checking that a value was given, in the same way and
with a message worded like the ones the other options use.

While at it, let's also set the environment variable before advancing
past the option, instead of advancing first and using `(*argv)[0]`, so
that this option looks like the other ones.

Note that all the in-tree callers passing `--shallow-file` to a `git`
subprocess always pass a value after it, so they are not affected. In
`upload-pack.c` that value is an empty string, which is still accepted.

Signed-off-by: Christian Couder <christian.couder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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