pathspec: honor `PATHSPEC_PREFIX_ORIGIN` with empty prefix

pathspec: honor `PATHSPEC_PREFIX_ORIGIN` with empty prefix

Previous to commit 5d8f084a5 (pathspec: simpler logic to prefix original
pathspec elements, 2017-01-04), we were always using the computed
`match` variable to perform pathspec matching whenever
`PATHSPEC_PREFIX_ORIGIN` is set. This is for example useful when passing
the parsed pathspecs to other commands, as the computed `match` may
contain a pathspec relative to the repository root. The commit changed
this logic to only do so when we do have an actual prefix and when
literal pathspecs are deactivated.

But this change may actually break some commands which expect passed
pathspecs to be relative to the repository root. One such case is `git
add --patch`, which now fails when using relative paths from a
subdirectory. For example if executing "git add -p ../foo.c" in a
subdirectory, the `git-add--interactive` command will directly pass
"../foo.c" to `git-ls-files`. As ls-files is executed at the
repository's root, the command will notice that "../foo.c" is outside
the repository and fail.

Fix the issue by again using the computed `match` variable when
`PATHSPEC_PREFIX_ORIGIN` is set and global literal pathspecs are
deactivated. Note that in contrast to previous behavior, we will now
always call `prefix_magic` regardless of whether a prefix is actually
set. But this is the right thing to do: when the `match` variable has
been resolved to the repository's root, it will be set to an empty
string. When passing the empty string directly to other commands, it
will result in a warning regarding deprecated empty pathspecs. By always
adding the prefix magic, we will end up with at least the string
":(prefix:0)" and thus avoid the warning.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Acked-by: Brandon Williams <bmwill@google.com>
Reviewed-by: Duy Nguyen <pclouds@gmail.com>