Our "util.h" header is a grabbag of various different functions, where
many don't have a clear group they belong to. Our set of allocator
functions though can be clearly singled out as a single group of
functions that always belongs together. Furthermore, we will need to
implement additional functions relating to our allocators subsystem when
moving to pluggable allocators. Thus, we should just move these
functions into their own "alloc" module.

Right now, the standard allocator is being declared as part of the
"util.h" header as a set of inline functions. As with the crtdbg
allocator functions, these inline functions make it hard to convert to
function pointers for our allocators.

Create a new "stdalloc" module containing our standard allocations
functions to split these out. Convert the existing allocators to macros
which make use of the stdalloc functions.

Currently, the `git__free` function is being defined in a single place,
only, disregarding whether we use our standard allocators or the crtdbg
allocators. This makes it a bit harder to convert our code base to use
pluggable allocators, and furthermore makes the border between our two
allocators a bit more blurry.

Implement a separate `git__crtdbg__free` function for the crtdbg
allocator in order to completely separate both allocator
implementations.

The diff driver truncates the hunk header text to 80 bytes, which can truncate
4-byte Unicode characters and introduce garbage characters in the diff
output. This change sanitizes the hunk header before it is displayed.

This mirrors the test in git: https://github.com/git/git/blob/master/t/t4025-hunk-header.sh

Closes https://github.com/libgit2/rugged/issues/716

While "util.h" declares the macro `git__tolower`, which simply resorts
to tolower(3P) on Unix-like systems, the <ctype.h> header is only being
included in "util.c". Thus, anybody who has included "util.h" without
having <ctype.h> included will fail to compile as soon as the macro is
in use.

Furthermore, we can clean up additional includes in "util.c" and simply
replace them with an include for "common.h".

use consistent names for the #include / #define header guard pattern.

Introduce `git_prefixncmp` that will search up to the first `n`
characters of a string to see if it is prefixed by another string.
This is useful for examining if a non-null terminated character
array is prefixed by a particular substring.

Consolidate the various implementations of `git__prefixcmp` around a
single core implementation and add some test cases to validate its
behavior.

Strict aliasing rules dictate that for most data types, you are not
allowed to cast them to another data type and then access the casted
pointers. While this works just fine for most compilers, technically we
end up in undefined behaviour when we hurt that rule.

Our current refcounting code makes heavy use of casting and thus
violates that rule. While we didn't have any problems with that code,
Travis started spitting out a lot of warnings due to a change in their
toolchain. In the refcounting case, the code is also easy to fix:
as all refcounting-statements are actually macros, we can just access
the `rc` field directly instead of casting.

There are two outliers in our code where that doesn't work. Both the
`git_diff` and `git_patch` structures have specializations for generated
and parsed diffs/patches, which directly inherit from them. Because of
that, the refcounting code is only part of the base structure and not of
the children themselves. We can help that by instead passing their base
into `GIT_REFCOUNT_INC`, though.

Next to including several files, our "common.h" header also declares
various macros which are then used throughout the project. As such, we
have to make sure to always include this file first in all
implementation files. Otherwise, we might encounter problems or even
silent behavioural differences due to macros or defines not being
defined as they should be. So in fact, our header and implementation
files should make sure to always include "common.h" first.

This commit does so by establishing a common include pattern. Header
files inside of "src" will now always include "common.h" as its first
other file, separated by a newline from all the other includes to make
it stand out as special. There are two cases for the implementation
files. If they do have a matching header file, they will always include
this one first, leading to "common.h" being transitively included as
first file. If they do not have a matching header file, they instead
include "common.h" as first file themselves.

This fixes the outlined problems and will become our standard practice
for header and source files inside of the "src/" from now on.

Some of our header files are not included at all by any of their
implementing counter-parts. Including them inside of these files leads
to some compile errors mostly due to unknown types because of missing
includes. But there's also one case where a declared function does not
match the implementation's prototype.

Fix all these errors by fixing up the prototype and adding missing
includes. This is preparatory work for fixing up missing includes in the
implementation files.