Also matches git.

This matches git.

Not that it matters much anyway but...

docs: fix statement about tab width

diff: fix enum value being out of allowed range

The libgit2 project mostly follows the coding style of git and thus
the linux project. While those two projects use a recommended tab width
of eight spaces, we instruct users to set their editor's tab width to
four spaces. Fix this to say eight instead.

The C89 standard states in §6.7.2.2 "Enumeration specifiers":

> The expression that defines the value of an enumeration constant shall
> be an integer constant expression that has a value representable as an
> int.

On most platforms, this effectively limits the range to a 32 bit signed
integer. The enum `git_diff_option_t` though recently gained an entry
`GIT_DIFF_INDENT_HEURISTIC = (1u << 31)`, which breaks this limit.

Fix the issue by using a gap in `git_diff_option_t`'s enum values. While
this has the benefit of retaining our API, it may break applications
which do not get recompiled after upgrading libgit2. But as we are
bumping the soversion on each release anyway and thus force a recompile
of dependents, this is not a problem.

pack: rename `git_packfile_stream_free`

Stop leaking the memory

The function `git_packfile_stream_free` frees all state of the packfile
stream without freeing the structure itself. This naming makes it hard
to spot whether it will try to free the pointer itself or not, causing
potential future errors. Due to this reason, we have decided to name a
function freeing state without freeing the actual struture a "dispose"
function.

Rename `git_packfile_stream_free` to `git_packfile_stream_dispose` as a
first example following this rule.

Fix stash save bug with fast path index check