This function has previously been implemented in Windows-specific path
handling code as `path__is_dirsep`. As we will need this functionality
in other parts, extract the logic into "path.h" alongside with a
non-Windows implementation.

Several functions to retrieve variables from a repository only return
immutable values, which allows us to actually constify the passed-in
repository parameter. Do so to help a later patch, which will only have
access to a constant repository.

Our current configuration logic is completely oblivious of any
repository, but only cares for actual file paths. Unfortunately, we are
forced to break this assumption by the introduction of conditional
includes, which are evaluated in the context of a repository. Right now,
only one conditional exists with "gitdir:" -- it will only include the
configuration if the current repository's git directory matches the
value passed to "gitdir:".

To support these conditionals, we have to break our API and make the
repository available when opening a configuration file. This commit
extends the `open` call of configuration backends to include another
repository and adjusts existing code to have it available. This includes
the user-visible functions `git_config_add_file_ondisk` and
`git_config_add_backend`.

The logic inside this function will be required later on, when
implementing conditional includes. Extract it into its own function to
ease the implementation.

To implement conditional includes, we require certain information from
the repository in whose context a file is being parsed. To have access
to this, pass down the repository to the parser to make it available.

Upstream git.git has implemented the ability to include other
configuration files based on conditions. Right now, this only includes
the ability to include a file based on the gitdir-location of the
repository the currently parsed configuration file belongs to. This
commit implements handling these conditional includes for the
case-sensitive "gitdir" condition.

Next to the "gitdir" conditional for including other configuration
files, there's also a "gitdir/i" conditional. In contrast to the former
one, path matching with "gitdir/i" is done case-insensitively. This
commit implements the case-insensitive condition.