int-conversion

While GCC enables int-conversion warnings by default, it will currently
only warn about such errors even in case where "-DENABLE_WERROR=ON" has
been passed to CMake. Explicitly enable int-conversion warnings by using
our `ENABLE_WARNINGS` macro, which will automatically use
"-Werror=int-conversions" in case it has been requested by the user.

GCC warns by default when implicitly converting integers to pointers or
the other way round, and commit fa48d2ea (vector: do not malloc
0-length vectors on dup, 2018-09-26) introduced such an implicit
conversion into our vector tests. While this is totally fine in this
test, as the pointer's value is never being used in the first place, we
can trivially avoid the warning by instead just inserting a pointer for
a variable allocated on the stack into the vector.

cmake: enable new quoted argument policy CMP0054

fix check if blob is uninteresting when inserting tree to packbuilder

Quoting from CMP0054's documentation:

    Only interpret if() arguments as variables or keywords when
    unquoted.

    CMake 3.1 and above no longer implicitly dereference variables or
    interpret keywords in an if() command argument when it is a Quoted
    Argument or a Bracket Argument.

    The OLD behavior for this policy is to dereference variables and
    interpret keywords even if they are quoted or bracketed. The NEW
    behavior is to not dereference variables or interpret keywords that
    have been quoted or bracketed.

The previous behaviour could be quite unexpected. Quoted arguments might
be expanded in case where the value of the argument corresponds to a
variable. E.g. `IF("MONKEY" STREQUAL "MONKEY")` would have been expanded
to `IF("1" STREQUAL "1")` iff `SET(MONKEY 1)` was set. This behaviour
was weird, and recent CMake versions have started to complain about this
if they see ambiguous situations. Thus we want to disable it in favor of
the new behaviour.

Our CMake coding style dictates that there should be no space between
`IF` and its opening `(`. Adjust our policy statements to honor this
style.

Documentation fixups

The underlying code uses GIT_CONFIG_FILENAME_GLOBAL, which is .gitconfig.

Blobs that have been marked as uninteresting should not be inserted into packbuilder
when inserting a tree. The check as to whether a blob was uninteresting looked at
the status for the tree itself instead of the blob.

This could cause significantly larger packfiles.

CI: refactoring

Don't stop on test failures; run all the tests, even when a test fails.

Similar to the way we parse the ctest output on POSIX systems, do the
same on Windows.  This allows us to append the `-r` flag to clar after
we've identified the command to run.

In-memory configuration

Now that we have abstracted away how to store and retrieve config
entries, it became trivial to implement a new in-memory backend by
making use of this. And thus we do so.

This commit implements a new read-only in-memory backend that can parse
a chunk of memory into a `git_config_backend` structure.

Right now, the config file code requires us to pass in its backend to
the config entry iterator. This is required with the current code, as
the config file backend will first create a read-only snapshot which is
then passed to the iterator just for that purpose. So after the iterator
is getting free'd, the code needs to make sure that the snapshot gets
free'd, as well.

By now, though, we can easily refactor the code to be more efficient and
remove the reverse dependency from iterator to backend. Instead of
creating a read-only snapshot (which also requires us to re-parse the
complete configuration file), we can simply duplicate the config entries
and pass those to the iterator. Like that, the iterator only needs to
make sure to free the duplicated config entries, which is trivial to do
and clears up memory ownership by a lot.

Access to the config entries is now completely done via the modules
function interface and no caller messes with the struct's internals. We
can thus completely move the structure declarations into the
implementation file so that nobody even has a chance to mess with the
members.

Instead of directly calling `git_atomic_inc` in users of the config
entries store, provide a `git_config_entries_incref` function to further
decouple the interfaces. Convert the refcount to a `git_refcount`
structure while at it.

The nice thing about our `git_config_iterator` interfaces is that nobody
needs to know anything about the implementation details. All that is
required is to obtain the iterator via any backend and then use it by
executing generic functions. We can thus completely internalize all the
implementation details of how to iterate over entries into the config
entries store and simply create such an iterator in our config file
backend when we want to iterate its entries. This further decouples the
config file backend from the config entries store.

The code accessing config entries in the `git_config_entries` structure
is still much too intimate with implementation details, directly
accessing the maps and handling indices. Provide two new functions to
get config entries from the internal map structure to decouple the
interfaces and use them in the config file code.

The function `git_config_entries_get` will simply look up the entry by
name and, in the case of a multi-value, return the last occurrence of
that entry. The second function, `git_config_entries_get_unique`, will
only return an entry if it is unique and not included via another
configuration file. This one is required to properly implement write
operations for single entries, as we refuse to write to or delete a
single entry if it is not clear which one was meant.

The previous commit simply moved all code that is required to handle
config entries to a new module without yet adjusting any of the function
and structure names to help readability. We now rename things
accordingly to have a common "git_config_entries" entries instead of the
old "diskfile_entries" one.

The configuration entry store that is used for configuration files needs
to keep track of all entries in two different structures:

- a singly linked list is being used to be able to iterate through
  configuration files in the order they have been found

- a string map is being used to efficiently look up configuration
  entries by their key

This store is thus something that may be used by other, future backends
as well to abstract away implementation details and iteration over the
entries.

Pull out the necessary functions from "config_file.c" and moves them
into their own "config_entries.c" module. For now, this is simply moving
over code without any renames and/or refactorings to help reviewing.

The implementation for config file snapshots has an unnecessary
redirection from `config_snapshot` to `git_config_file__snapshot`.
Inline the call to `git_config_file__snapshot` and remove it.

The header "config_file.h" has a list of inline-functions to access the
contents of a config backend without directly messing with the struct's
function pointers. While all these functions are called
"git_config_file_*", they are in fact completely backend-agnostic and
don't care whether it is a file or not. Rename all the function to
instead be backend-agnostic versions called "git_config_backend_*" and
rename the header to match.

The function `git_config_file_normalize_section` is never being used in
any file different than "config.c", but it is implemented in
"config_file.c". Move it over and make the symbol static.

As a last step to make variables and structures more backend agnostic
for our `git_config` structure, rename local variables to not be called
`file` anymore.

Some warnings

index: release the snapshot instead of freeing the index

"warning: values of type 'OSStatus' should not be used as format arguments; add an explicit cast to 'int' instead [-Wformat]"

online::clone: free url and username before resetting

Before resetting the url and username, ensure that we free them in case
they were set by environment variables.

git_remote_prune to be O(n  * logn)

Same as with the previous commit, the `file_internal` struct is used to
keep track of all the backends that are added to a `git_config` struct.
Rename it to `backend_internal` and rename its `file` member to
`backend` to make the implementation more backend-agnostic.