For sections we will still use the existing one even if the case disagrees, but
the variable always gets written with the case given by the caller.

While most parts of a configuration key are case-insensitive, we should still be
case-preserving and write down whatever string the caller provided.

example-log: add support for --log-size

Implement a diff indent heuristic

Ask for SSH credentials again when passphrase is wrong

examples: remove Makefile

When trying to decode the private key it looks like LibSSH2 returns a
LIBSSH2_ERROR_PUBLICKEY_UNVERIFIED when the passphrase is incorrect.

Back in the days when libgit2 was still young, libgit2 was using plain
Makefiles as build infrastructure. We later changed that to instead use
the CMake build system to make cross-platform development easier. In the
process, we forgot to remove the Makefile from our examples directory,
which is fixed by this commit here. Furthermore, remove the accompanying
.gitignore file, which ignores build outputs. As we do out-of-tree
builds only nowadays, no output is generated in that directory anymore.

cmake: use project-relative binary and source directories

Due to our split of CMake files into multiple modules, we had to replace
some uses of the `${CMAKE_CURRENT_SOURCE_DIR}` and
`${CMAKE_CURRENT_BINARY_DIR}` variables and replace them with
`${CMAKE_SOURCE_DIR}` and `${CMAKE_BINARY_DIR}`. This enabled us to
still be able to refer to top-level files when defining build
instructions inside of a subdirectory.

When replacing all variables, it was assumed that the absolute set of
variables is always relative to the current project. But in fact, this
is not the case, as these variables always point to the source and
binary directory as given by the top-levl project. So the change
actually broke the ability to include libgit2 directly as a subproject,
as source files cannot be found anymore.

Fix this by instead using project-specific source and binary directories
with `${libgit2_SOURCE_DIR}` and `${libgit2_BINARY_DIR}`.

Fix compilation for CMake versions 3.0.x where x >= 1

`inline` is not portable enough, and the `xdiff` code doesn't import
the `GIT_INLINE` macro. So introduce a new `XDL_INLINE` macro (with
the same definition as `GIT_INLINE`). Use the new macro to inline two
functions in `xdiffi.c`.

Apparently policy CMP0015 was added in CMake 3.1. With CMake 3.0.2,
the build was failing with

    CMake Error at CMakeLists.txt:18 (CMAKE_POLICY):
      Policy "CMP0051" is not known to this version of CMake.

This patch makes it work.

Cleaned up the PR to address styling issues.

cmake: use static dependencies when building static libgit2

cmake: fix linking in Xcode with object libraries only

transports: smart: fix memory leak when skipping symbolic refs

tests: checkout::tree: verify status entrycount changes on chmod

CMake is unable to generate a correct Xcode project when trying to link
libraries with only object libraries as its input. As our new build
infrastructure makes heavy use of object libraries now, this affects our
libgit2 library target, as well, leading to linking errors.

Fix the issue by adding a dummy file to the libgit2 objects. As we
always have the "features.h" header ready which contains defines only,
we can simply link it into the resulting library without any effect
whatsoever. This fixes building with Xcode.

While we verify that we have no mode changes after calling
`git_checkout_tree`, we do not verify that the `p_chmod` calls actually
resulted in a changed entry. While we should assume that this works due
to separate tests for the status list, we should test for the change
being listed to avoid programming errors in the test.

There are multiple locations where we have the same code to check
whether the count of status list entries of a repository matches an
expected number. Extract that into a common function.

Checkout typechange-only deltas

When we setup the revision walk for negotiating references with a
remote, we iterate over all references, ignoring tags and symbolic
references. While skipping over symbolic references, we forget to free
the looked up reference, resulting in a memory leak when the next
iteration simply overwrites the variable.

Fix that issue by freeing the reference at the beginning of each
iteration and collapsing return paths for error and success.

Plug some leaks in curl's proxy handling

Test that we can successfully force checkout a target when the file
contents are identical, but the mode has changed.

On Windows, we do not support file mode changes, so do not test
for type changes between the disk and tree being checked out.

We could have false positives since the on-disk file can only have
an (effective) mode of 0100644 since NTFS does not support executable
files.  If the tree being checked out did have an executable file,
we would erroneously decide that the file on disk had been changed.

Fix Issue #4047 Check return codes and free objects

travis: add custom apt sources

remote: add typedef to normalize push_update_reference callback

refs: do not use peeled OID if peeling to a tag

Move back to Travis's VM infrastructure for efficiency.

When performing a forced checkout, treat files as modified when the
workdir or the index is identical except for the mode.  This ensures
that force checkout will update the mode to the target.  (Apply this
check for regular files only, if one of the items was a file and the
other was another type of item then this would be a typechange and
handled independently.)

Very many callbacks in libgit2 have some sort of typedef to normalize the name at git_<name_of_operation>_cb. Add a typedef for push_update_references in the remote so the name follows the same conventions.