`git_buf_sanitize` is called with user-input, and wants to sanity-check
that input.  Allow it to return a value if the input was malformed in a
way that we cannot cope.

We currently don't check for out-of-memory situations on exiting
`format_binary` and, as a result, may return a partially filled buffer.
Fix this by checking the buffer via `git_buf_oom`.

Calling abort(3P) in a library is rather rude and shouldn't happen, as
we effectively prohibit any corrective actions made by the application
linking to it. We thus shouldn't call it at all, but instead use our new
`GIT_ASSERT` macros.

Remove the call to abort(3P) in case a diff delta has an unexpected type
to fix this.

When printing the diff to a `FILE *` handle, we neither check the return
value of fputc(3P) nor the one of fwrite(3P). As a result, we'll
silently return successful even if we didn't print anything at all.
Futhermore, the arguments to fwrite(3P) are reversed: we have one item
of length `content_len`, and not `content_len` items of one byte.

Fix both issues by checking return values as well as reversing the
arguments to fwrite(3P).

We've accumulated quite some functions which are never used outside of
their respective code unit, but which are lacking the `static` keyword.
Add it to reduce their linkage scope and allow the compiler to optimize
better.

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.

Git is generating patch-id using a stripped down version of a patch
where hunk header and index information are not present.

Signed-off-by: Gregory Herrero <gregory.herrero@oracle.com>

Add a new 'print_index' flag to let the caller decide whether or not
'index <oid>..<oid>' should be printed.
Since patch id needs not to have index when hashing a patch, it will be
useful soon.

Signed-off-by: Gregory Herrero <gregory.herrero@oracle.com>