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.

The reader machinery will be extended to handle conditional includes.
The only conditions that currently exist all match the against the git
directory of the repository the config file belongs to. As such, we need
to have access to the repository when reading configuration files to
properly handle these conditions.

One specialty of thes conditional includes is that the actual pattern
may also be a relative pattern starting with "./". In this case, we have
to match the pattern against the path relative to the config file which
is currently being parsed. So besides the repository, we also have to
pass down the path to the current config file that is being parsed.

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.

…ncludes