diff: improve scaling of filenames in diffstat to handle UTF-8 chars

diff: improve scaling of filenames in diffstat to handle UTF-8 chars

The `show_stats()` function tries to scale the filenames in the diffstat to
ensure they don't exceed the given `name-width`. It does so by calculating
the "display width" of the characters to be dropped, but then advances the
filename pointer by that number of bytes.

However, the "display width" of a character is not always equal to its byte
count. The result is that sometimes, when displaying UTF-8 characters,
filenames exceed the given `name-width`, and frequently the bytes of the
UTF-8 characters are truncated.

The following is an example of the issue, where the 2 files are "HelloHi" and
"Hello你好", and `name-width=6`:

    ...oHi | 0
    ...<BD><A0>好 | 0

Make the filename pointer move by the actual number of bytes of the
characters to drop from the filename, rather than their display width, using
the `utf8_width()` function.

Force `len` to not be less than 0 (this happens if the given `name-width` is
2 or less), otherwise an infinite loop is entered.

Signed-off-by: LorenzoPegorari <lorenzo.pegorari2002@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>