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
```

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

Test with some postimages that actually grow/shrink from the
original, adding new lines or removing them.  (Also do so without
context to ensure that we can add/remove from a non-zero part of
the line vector.)

Handle prefixes (in terms of number of path components) for patch
parsing.

Patches can now come from a variety of sources - either internally
generated (from diffing two commits) or as the results of parsing
some external data.