This adds an option which will check if a diff is applicable without
actually applying it; equivalent to git apply --check.

apply: Fix a patch corruption related to EOFNL handling

Introduce an unit test to validate that git_apply__patch() properly
handles EOFNL changes in case of patches with several hunks.

Introduce an unit test to validate if git_apply_to_tree() fails when an
applied patch adds new files.

Previously, we would fail to correctly truncate the source buffer
if the source has more than one line and ends with a non-newline
character. In the following call, we thus truncate the source
string in the middle of the second line. Without the bug fixed,
we would successfully apply the patch to the source and return
success. With the overflow being fixed, we should return an
error now.

Use the new-style index names throughout our own codebase.

Currently, the "apply_helper" functions used for testing the apply logic are all
statically defined in the "apply_helpers.h" header file. This may lead to
warnings from the compiler in case where this header file is included, but not
all functions it brings along are used in the compilation unit where it has been
included into.

Fix these potential warnings by moving the implementation into its own
compilation unit "apply_helpers.c".

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
```