diff.c: use utf8_strwidth() to count display width

When unicode filenames (encoded in UTF-8) are used, the visible width on the screen is not the same as strlen(). For example, `git log --stat` may produce an output like this: [snip the header] Arger.txt | 1 + Ärger.txt | 1 + 2 files changed, 2 insertions(+) A side note: the original report was about cyrillic filenames. After some investigations it turned out that a) This is not a problem with "ambiguous characters" in unicode b) The same problem exists for all unicode code points (so we can use Latin based Umlauts for demonstrations below) The 'Ä' takes the same space on the screen as the 'A'. But needs one more byte in memory, so the the `git log --stat` output for "Arger.txt" (!) gets mis-aligned: The maximum length is derived from "Ärger.txt", 10 bytes in memory, 9 positions on the screen. That is why "Arger.txt" gets one extra ' ' for aligment, it needs 9 bytes in memory. If there was a file "Ö", it would be correctly aligned by chance, but "Öhö" would not. The solution is of course, to use utf8_strwidth() instead of strlen() when dealing with the width on screen. And then there is another problem, code like this: strbuf_addf(&out, "%-*s", len, name); (or using the underlying snprintf() function) does not align the buffer to a minimum of len measured in screen-width, but uses the memory count. One could be tempted to wish that snprintf() was UTF-8 aware. That doesn't seem to be the case anywhere (tested on Linux and Mac), probably snprintf() uses the "bytes in memory"/strlen() approach to be compatible with older versions and this will never change. The basic idea is to change code in diff.c like this strbuf_addf(&out, "%-*s", len, name); into something like this: int padding = len - utf8_strwidth(name); if (padding < 0) padding = 0; strbuf_addf(&out, " %s%*s", name, padding, ""); The real change is slighty bigger, as it, as well, integrates two calls of strbuf_addf() into one. Tests: Two things need to be tested: - The calculation of the maximum width - The calculation of padding The name "textfile" is changed into "tëxtfilë", both have a width of 8. If strlen() was used, to get the maximum width, the shorter "binfile" would have been mis-aligned: binfile | [snip] tëxtfilë | [snip] If only "binfile" would be renamed into "binfilë": binfilë | [snip] textfile | [snip] In order to verify that the width is calculated correctly everywhere, "binfile" is renamed into "binfilë", giving 1 bytes more in strlen() "tëxtfile" is renamed into "tëxtfilë", 2 byte more in strlen(). The updated t4012-diff-binary.sh checks the correct aligment: binfilë | [snip] tëxtfilë | [snip] Reported-by: Alexander Meshcheryakov <alexander.s.m@gmail.com> Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de> Signed-off-by: Torsten Bögershausen <tboegi@web.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>