git: make super-prefix option

git: make super-prefix option

Add a super-prefix environment variable 'GIT_INTERNAL_SUPER_PREFIX'
which can be used to specify a path from above a repository down to its
root.  When such a super-prefix is specified, the paths reported by Git
are prefixed with it to make them relative to that directory "above".
The paths given by the user on the command line
(e.g. "git subcmd --output-file=path/to/a/file" and pathspecs) are taken
relative to the directory "above" to match.

The immediate use of this option is by commands which have a
--recurse-submodule option in order to give context to submodules about
how they were invoked.  This option is currently only allowed for
builtins which support a super-prefix.

Signed-off-by: Brandon Williams <bmwill@google.com>
Reviewed-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>