Move to the `git_error` name in the internal API for error-related
functions.

This change fixes -Wmaybe-uninitialized and -Wdeprecated-declarations
warnings on Linux builds

tests: 🌀 address two null argument instances

Handle two null argument cases that occur in the unit tests.
One is in library code, the other is in test code.

Detected by running unit tests with undefined behavior sanitizer:
```bash
 # build
mkdir build && cd build
cmake -DBUILD_CLAR=ON -DCMAKE_C_FLAGS="-fsanitize=address \
-fsanitize=undefined -fstack-usage -static-libasan" ..
cmake --build .

 # run with asan
ASAN_OPTIONS="allocator_may_return_null=1" ./libgit2_clar
...
............../libgit2/src/apply.c:316:3: runtime error: null pointer \
passed as argument 1, which is declared to never be null
...................../libgit2/tests/apply/fromfile.c:46:3: runtime \
error: null pointer passed as argument 1, which is declared to never be null
```

Ensure that we can add a file back after it's been removed.  Update the
renamed/deleted validation in application to not apply to deltas that
are adding files to support this.

Ensure that we cannot modify a file after it's been renamed out of the
way.  If multiple deltas exist for a single path, ensure that we do not
attempt to modify a file after it's been renamed out of the way.

To support this, we must track the paths that have been removed or
renamed; add to a string map when we remove a path and remove from the
string map if we recreate a path.  Validate that we are not applying to
a path that is in this map, unless the delta is a rename, since git
supports renaming one file to two different places in two different
deltas.

Further, test that we cannot apply a modification delta to a path that
will be created in the future by a rename (a path that does not yet
exist.)

git allows a patch file to contain multiple deltas to the same file:
although it does not produce files in this format itself, this could
be the result of concatenating two different patch files that affected
the same file.

git apply behaves by applying this next delta to the existing postimage
of the file.  We should do the same.  If we have previously seen a file,
and produced a postimage for it, we will load that postimage and apply
the current delta to that.  If we have not, get the file from the
preimage.

Deltas containing exact renames are special; they simple indicate that a
file was renamed without providing additional metadata (like the
filemode).  Teach the reader to provide the file mode and use the
preimage's filemode in the case that the delta does not provide one.)

Introduce a callback to patch application that allows consumers to
cancel hunk application.