docs: add documentation for our coding style

MSVC: Enable Control Flow Guard (CFG)

git__hexdump: better mimic `hexdump -C`

This feature requires Visual Studio 2015 (MSVC_VERSION = 1900) or later. As the
minimum required CMake version is currently less than 3.7, GREATER_EQUAL is not
available to us and we must invert the result of the LESS operator.

Feature: Allow blame to ignore whitespace change

deps: ntlmclient: use htobe64 on NetBSD too

For years, we've repeatedly had confusion about what our actual coding
style is not only for newcomers, but also across the core contributors.
This can mostly be attributed to the fact that we do not have any coding
conventions written down. This is now a thing of the past with the
introduction of a new document that gives an initial overview of our
style and most important best practices for both our C codebase as well
as for CMake.

While the proposed coding style for our C codebase should be rather
uncontroversial, the coding style for CMake might be. This can be
attributed to multiple facts. First, the CMake code base doesn't really
have any uniform coding style and is quite outdated in a lot of places.
Second, the proposed coding style actually breaks with our existing one:
we currently use all-uppercase function names and variables, but the
documented coding style says we use all-lowercase function names but
all-uppercase variables.

It's common practice in CMake to write variables in all upper-case, and
in fact all variables made available by CMake are exactly that. As
variables are case-sensitive in CMake, we cannot and shouldn't break
with this. In contrast, function calls are case insensitive, and modern
CMake always uses all-lowercase ones. I argue we should do the same to
get in line with other codebases and to reduce the likelihood of
repetitive strain injuries.

So especially for CMake, the proposed coding style says something we
don't have yet. I'm fine with that, as the document explicitly says that
it's what we want to have and not what we have right now.

sysdir: remove unused git_sysdir_get_str

Fix typo causing removal of symbol 'git_worktree_prune_init_options'

Commit 0b5ba0d7 replaced this function with an "option_init"
equivallent, but misspelled the replacement function. As a result, this
symbol has been missing from libgit2.so ever since.

pack: Improve error handling for get_delta_base()

repo::open: ensure we can open the repository

examples: additions and fixes

merge: cache negative cache results for similarity metrics

Update the test cases to check the `git_repository_open` return code.

add example for git commit
fix example for git add
add example for git push

Handle repository format v1

CMake: backend selection streamlining

refdb_fs: remove unused header file

This makes get_delta_base() return the error code as the return value
and the delta base as an out-parameter.

This change moves the responsibility of setting the error upon failures
of get_delta_base() to get_delta_base() instead of its callers. That
way, the caller chan always check if the return value is negative and
mark the whole operation as an error instead of using garbage values,
which can lead to crashes if the .pack files are malformed.

patch: correctly handle mode changes for renames

gitignore: clean up patterns from old times

README.md: update build matrix to reflect our latest releases

We're currently doing unnecessary work to auto-detect backends even if
the functionality is disabled altogether. Let's fix this by removing the
extraneous FOO_BACKEND variables, instead letting auto-detection modify
the variable itself.

Release v1.0

When computing renames, we cache the hash signatures for each of the
potentially conflicting entries so that we do not need to repeatedly
read the file and can at least halfway efficiently determine whether two
files are similar enough to be deemed a rename. In order to make the
hash signatures meaningful, we require at least four lines of data to be
present, resulting in at least four different hashes that can be
compared. Files that are deemed too small are not cached at all and
will thus be repeatedly re-hashed, which is usually not a huge issue.

The issue with above heuristic is in case a file does _not_ have at
least four lines, where a line is anything separated by a consecutive
run of "\n" or "\0" characters. For example "a\nb" is two lines, but
"a\0\0b" is also just two lines. Taken to the extreme, a file that has
megabytes of consecutive space- or NUL-only may also be deemed as too
small and thus not get cached. As a result, we will repeatedly load its
blob, calculate its hash signature just to finally throw it away as we
notice it's not of any value. When you've got a comparitively big file
that you compare against a big set of potentially renamed files, then
the cost simply expodes.

The issue can be trivially fixed by introducing negative cache entries.
Whenever we determine that a given blob does not have a meaningful
representation via a hash signature, we store this negative cache marker
and will from then on not hash it again, but also ignore it as a
potential rename target. This should help the "normal" case already
where you have a lot of small files as rename candidates, but in the
above scenario it's savings are extraordinarily high.

To verify we do not hit the issue anymore with described solution, this
commit adds a test that uses the exact same setup described above with
one 50 megabyte blob of '\0' characters and 1000 other files that get
renamed. Without the negative cache:

$ time ./libgit2_clar -smerge::trees::renames::cache_recomputation >/dev/null
real    11m48.377s
user    11m11.576s
sys     0m35.187s

And with the negative cache:

$ time ./libgit2_clar -smerge::trees::renames::cache_recomputation >/dev/null
real    0m1.972s
user    0m1.851s
sys     0m0.118s

So this represents a ~350-fold performance improvement, but it obviously
depends on how many files you have and how big the blob is. The test
number were chosen in a way that one will immediately notice as soon as
the bug resurfaces.

The gitignore file currently has a lot of patterns for files that we
shouldn't write anymore since we have migrated to CMake, as everybody is
expected to do out-of-source builds anyway. Let's remove them.

When generating a patch for a renamed file whose mode bits have changed
in addition to the rename, then we currently fail to parse the generated
patch. Furthermore, when generating a diff we output mode bits after the
similarity metric, which is different to how upstream git handles it.

Fix both issues by adding another state transition that allows
similarity indices after mode changes and by printing mode changes
before the similarity index.

refdb_backend: improve callback documentation

credentials: provide backcompat for opaque structs