abbrev: allow extending beyond 32 chars to disambiguate

abbrev: allow extending beyond 32 chars to disambiguate

When you have two or more objects with object names that share more
than 32 letters in an SHA-1 repository, find_unique_abbrev() fails
to show disambiguation.

To see how many leading letters of a given full object name is
sufficiently unambiguous, the algorithm starts from a initial
length, guessed based on the estimated number of objects in the
repository, and see if another object that shares the prefix, and
keeps extending the abbreviation.  The loop stops at GIT_MAX_RAWSZ,
which is counted as the number of bytes, since 5b20ace6 (sha1_name:
unroll len loop in find_unique_abbrev_r(), 2017-10-08); before that
change, it extended up to GIT_SHA1_HEXSZ, which meant to stop at the
end of hexadecimal SHA-1 object name.

Because the hexadecimal object name passed to the function is
NUL-terminated, and this fact is used to correctly terminate the
loop that scans for the first difference earlier in the function,
use it to make sure we do not increment the .cur_len member beyond
the end of the string.

Noticed-by: Jon Forrest <nobozo@gmail.com>
Helped-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>