diff_unique_abbrev(): document its assumption and limitation

diff_unique_abbrev(): document its assumption and limitation

This function is used to add "..." to displayed object names in
"diff --raw --abbrev[=<n>]" output.  It bases its behaviour on an
untold assumption that the abbreviation length requested by the
caller is "reasonble", i.e. most of the objects will abbreviate
within the requested length and the resulting length would never
exceed it by more than a few hexdigits (otherwise the resulting
columns would not align).  Explain that in a comment.

Signed-off-by: Junio C Hamano <gitster@pobox.com>