git add -i: Fix parsing of abbreviated hunk headers

git add -i: Fix parsing of abbreviated hunk headers

The unified diff format allows one-line ranges to be abbreviated
by omiting the size.  The hunk header "@@ -10,1 +10,1 @@" can be
expressed as "@@ -10 +10 @@", but this wasn't properly parsed in
all cases.

Such abbreviated hunk headers are generated when a one-line change
(add, remove or modify) appears without context; for example
because the file is a one-liner itself or because GIT_DIFF_OPTS
was set to '-u0'.  If the user then runs 'git add -i' and enters
the 'patch' command for that file, perl complains about undefined
variables.

Signed-off-by: Jean-Luc Herren <jlh@gmx.ch>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>