completion: suppress unwanted unescaping of `read`

completion: suppress unwanted unescaping of `read`

The function `__git_eread`, which reads the first line from the file,
calls the `read` builtin without passing the flag option `-r`.  When
the `read` builtin is called without the flag `-r`, it processes the
backslash escaping in the text that it reads.  For this reason, it is
generally considered the best practice to always use the `read`
builtin with flag `-r` unless one intensionally processes the
backslash escaping.  For the present case in git-prompt.sh, in fact,
all the occurrences of the calls of `__git_eread` intend to read the
literal content of the first lines.

To make it read the first line literally, pass the flag `-r` to the
`read` builtin in the function `__git_eread`.

Signed-off-by: Edwin Kofler <edwin@kofler.dev>
Signed-off-by: Koichi Murase <myoga.murase@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>