pkgconfig: fix handling of prefixes containing whitespaces

Our libgit2.pc.in file is quoting the `libdir` variable in our declared
"Libs:" line. The intention is to handle whitespaces here, but pkgconfig
already does so by automatically escaping whitespace with backslashes.
The correct thing to do is to instead quote the prefix, as this is the
one which is being substituted by CMake upon installation. As both
libdir and includedir will be expanded to "${prefix}/lib" and
"${prefix}/include", respectively, pkgconfig will also correctly escape
whitespaces.

Note that this will actually break when a user manually wants to
override libdir and includedir with a path containing whitespace. But
actually, this cannot be helped, as always quoting these variables will
actuall break the common case of being prefixed with "${prefix}". So we
just bail out here and declare this as unsupported out of the box.

Correct non-existent file references in `odb.h`

There are references to odb_backends.h when the file is actually named
odb_backend.h and in the sys folder.

git_treebuilder_write_with_buffer refactorings

While writing the tree inside of a buffer, we check whether the buffer
runs out of memory after each tree entry. While we set the error code as
soon as we detect the OOM situation, we happily proceed iterating over
the entries. This is not useful at all, as we will try to write into the
buffer repeatedly, which cannot work.

Fix this by exiting as soon as we are OOM.

The `git_tree_entry *entry` variable is defined twice inside of this
function. While this is not a problem currently, remove the shadowing
variable to avoid future confusion.

While we detect errors in `git_treebuilder_write_with_buffer`, we just
exit directly instead of freeing allocated memory. Fix this by
remembering error codes and skipping forward to the function's cleanup
code.

Fix memory leaks

The recent addition of an error code to `pass_whole_blame` in ff8d2eb1
(blame_git: check return value of object lookup, 2017-03-20) introduced
a spurious goto. Remove it.

git_futils: don't O_EXCL and O_TRUNC

sha1dc: perf improvements from upstream

inet_pton: don't assume addr families don't exist

Update SHA-1 collision detection code (cr-marcstevens/sha1collisiondetection)
to master to include performance improvements.

Worktree fixes

fsync all the things

Coverity fixes

Fix the documentation for git_cred_acquire_cb

Address family 5 might exist on some crazy system like Haiku.
Use `INT_MAX-1` as an unsupported address family.

`O_EXCL` and `O_TRUNC` are mutually exclusive flags to open(2); you can't
truncate a file if you're asserting that it can't exist in the first place.
Drop `O_TRUNC`.

git_cred_acquire_cb isn't using the standard @param and @return tags.
This is causing the generated documentation to not be formatted
properly.

While parsing patch header lines, we iterate over each line and check if
the line has trailing garbage. What we do not check though is that the
line is actually a line ending with a trailing newline.

Fix this by checking the return code of `parse_advance_expected_str`.

The `pack_entry_find_prefix` function receives a `git_rawobj` structure
as argument. While the function first initializes the structure to a
sensible state, Coverity is unable to correctly detect this, resulting
in a warning.

Fix this warning by initializing the object to all-zeroes before passing
it to the function.

While parsing section headers, we use a buffer to store the actual
section name. We do not check though if the buffer runs out of memory at
any stage. Do so.

The function `pass_whole_blame` performs an object lookup but does not
check if the lookup actually succeeds. Convert the function to return an
error code and check for it in the calling function.

README: Mention how to run tests

Fix typo in remote.h API

The OpenSSL library may require multiple locks to work correctly, where
it is the caller's responsibility to initialize and release the locks.
While we correctly initialized up to `n` locks, as determined by
`CRYPTO_num_locks`, we were repeatedly freeing the same mutex in our
shutdown procedure.

Fix the issue by freeing locks at the correct index.

Remove `map_free` macros

The symlink destination is always concatenated to the original path. Fix
this by using `git_buf_sets` instead of `git_buf_puts`.

Diff fixes

merge_driver: fix const-correctness for source getters