pool: Simplify implementation

cc @carlosmn

Better REUC generation when merging

Inserting new REUC entries can quickly become pathological given that
each insert unsorts the REUC vector, and both subsequent lookups *and*
insertions will require sorting it again before being successful.

To avoid this, we're switching to `git_vector_insert_sorted`: this keeps
the REUC vector constantly sorted and lets us use the `on_dup` callback
to skip an extra binary search on each insertion.

Fix docs typo geterr_clear -> giterr_clear

inttypes.h is built-in header file since MSVC 2013

cmake: split sources into original paths for Xcode and MSVC

xdiff: reference util.h in parent directory

merge: add GIT_MERGE_TREE_FAIL_ON_CONFLICT

The MSVC_SPLIT_SOURCES function is helpful for other IDEs, like Xcode,
and will split the source files up into their target directories,
instead of merely placing them all in a "Sources" directory.

Rename MSVC_SPLIT_SOURCES to IDE_SPLIT_SOURCES and enable it for Xcode.

Provide a new merge option, GIT_MERGE_TREE_FAIL_ON_CONFLICT, which
will stop on the first conflict and fail the merge operation with
GIT_EMERGECONFLICT.

Nanoseconds in the index: ignore for diffing

Although CMake will correctly configure include directories for us,
some people may use their own build system, and we should reference
`util.h` based on where it actually lives.

Although our index contains the literal time present in the index,
we do not read nanoseconds from disk, and thus we should not use
them in any comparisons, lest we always think our working directory
is dirty.

Guard this behind a `GIT_USE_NSECS` for future improvement.

Test that nanoseconds are round-tripped correctly when we read
an index file that contains them.  We should, however, ignore them
because we don't understand them, and any new entries in the index
should contain a `0` nsecs field, while existing preserving entries.

The reason is that the types defined in libgit2's inttypes.h collide with system inttypes.h
3rd party library header files may directly reference MSVC's built-in inttypes.h

Fixes #3476

config: add a ProgramData level

api: be explicit about our C linkage

This is where portable git stores the global configuration which we can
use to adhere to it even though git isn't quite installed on the system.

signature: Strip crud

revwalk: make commit list use 64 bits for time

Preserve modes from a conflict in `git_index_insert`

Fix pathological performance in ODB lookups

For most real use cases, repositories with alternates use them as main
object storage. Checking the alternate for objects before the main
repository should result in measurable speedups.

Because of this, we're changing the sorting algorithm to prioritize
alternates *in cases where two backends have the same priority*. This
means that the pack backend for the alternate will be checked before the
pack backend for the main repository *but* both of them will be checked
before any loose backends.

In the current implementation of ODB backends, each backend is tasked
with refreshing itself after a failed lookup. This is standard Git
behavior: we want to e.g. reload the packfiles on disk in case they have
changed and that's the reason we can't find the object we're looking
for.

This behavior, however, becomes pathological in repositories where
multiple alternates have been loaded. Given that each alternate counts
as a separate backend, a miss in the main repository (which can
potentially be very frequent in cases where object storage comes from
the alternate) will result in refreshing all its packfiles before we
move on to the alternate backend where the object will most likely be
found.

To fix this, the code in `odb.c` has been refactored as to perform the
refresh of all the backends externally, once we've verified that the
object is nowhere to be found.

If the refresh is successful, we then perform the lookup sequentially
through all the backends, skipping the ones that we know for sure
weren't refreshed (because they have no refresh API).

The on-disk pack backend has been adjusted accordingly: it no longer
performs refreshes internally.

CMake: be more explicit with python errors