convert: clarify line ending conversion warning

convert: clarify line ending conversion warning

The warning about converting line endings is extremely confusing. Its
two sentences each use the word "will" without specifying a timeframe,
which makes it sound like both sentences are referring to the same
timeframe. On top of that, it uses the term "original line endings"
without saying whether "original" means LF or CRLF.

Rephrase the warning to be clear about when the line endings will be
changed and what they will be changed to.

On a platform whose native line endings are not CRLF (e.g. Linux), the
"git add" step in the following sequence triggers the warning in
question:

$ git config core.autocrlf true
$ echo 'Hello world!' >hello.txt
$ git add hello.txt
warning: LF will be replaced by CRLF in hello.txt
The file will have its original line endings in your working directory

Signed-off-by: Alex Henrie <alexhenrie24@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>