Fixes issue where a changed binary file's content in the working tree isn't displayed correctly, instead showing an oid of zero, and with its path being reported incorrectly as "/dev/null".
Our file utils functions all have a "futils" prefix, e.g. `git_futils_touch`. One would thus naturally guess that their definitions and implementation would live in files "futils.h" and "futils.c", respectively, but in fact they live in "fileops.h". Rename the files to match expectations.
Move to the `git_error` name in the internal API for error-related functions.
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.
Include fixups
Pointers to general data should usually be used as a void pointer such that it is possible to hand in variables of a different pointer type without the need to cast. This is the same when creating patches from buffers, where the buffers may contain arbitrary data. Instead of requiring the caller to care whether his buffer is e.g. `char *` or `unsigned char *`, we should instead just accept a `void *`. This is also consistent in how we tread other types like for example `git_blob`, which also just has a void pointer as its raw contents.
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.