Fix "git apply" to correctly enforce "match at the beginning"

Fix "git apply" to correctly enforce "match at the beginning"

An earlier commit 4be6096 (apply --unidiff-zero: loosen sanity checks for
--unidiff=0 patches, 2006-09-17) made match_beginning and match_end
computed incorrectly.  If a hunk inserts at the beginning, old position
recorded at the hunk is line 0, and if a hunk changes at the beginning, it
is line 1.  The new test added to t4104 exposes that the old code did not
insist on matching at the beginning for a patch to add a line to an empty
file.

An even older 65aadb9 (apply: force matching at the beginning.,
2006-05-24) was equally wrong in that it tried to take hints from the
number of leading context lines, to decide if the hunk must match at the
beginning, but we can just look at the line number in the hunk to decide.

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