Fix git-stripspace to process correctly long lines and spaces.

Fix git-stripspace to process correctly long lines and spaces.

Now the implementation gets more memory to store completely
each line before removing trailing spaces, and does it right
when the last line of the file ends with spaces and no newline
at the end.

Function stripspace needs again to be non-static in order to call
it from "builtin-tag.c" and the upcoming "builtin-commit.c".
A new parameter skip_comments was also added to the stripspace
function to optionally strips every shell #comment from the input,
needed for doing this task on those programs.

Signed-off-by: Carlos Rica <jasampler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>