Documentation/rev-parse: quoting is required with --parseopt

When calling rev-parse --parseopt, as in the (now fixed) documented
example

  eval "$(echo "$OPTS_SPEC" | git rev-parse --parseopt -- "$@" || echo exit $?)"

the outermost quoting is required, as otherwise all runs of arbitrary
whitespace inside the resulting 'set -- ...' call would be collapsed
into a single space.

This was exposed as a result of our new use of cat <<\EOF since
47e9cd2 (parseopt: wrap rev-parse --parseopt usage for eval
consumption, 2010-06-12), but has always been a problem when handling
arguments containing e.g. newlines.

Point this out in the documentation, and in particular correct the
example that did not have the quotes.

Noticed-by: Joshua Jensen <jjensen@workspacewhiz.com>
Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rev-parse.txt

@@ -184,10 +184,13 @@ scripts the same facilities C builtins have. It works as an option normalizer
 (e.g. splits single switches aggregate values), a bit like `getopt(1)` does.
 
 It takes on the standard input the specification of the options to parse and
-understand, and echoes on the standard output a line suitable for `sh(1)` `eval`
+understand, and echoes on the standard output a string suitable for `sh(1)` `eval`
 to replace the arguments with normalized ones.  In case of error, it outputs
 usage on the standard error stream, and exits with code 129.
 
+Note: Make sure you quote the result when passing it to `eval`.  See
+below for an example.
+
 Input Format
 ~~~~~~~~~~~~
 
@@ -244,7 +247,7 @@ bar=      some cool option --bar with an argument
   An option group Header
 C?        option C with an optional argument"
 
-eval `echo "$OPTS_SPEC" | git rev-parse --parseopt -- "$@" || echo exit $?`
+eval "$(echo "$OPTS_SPEC" | git rev-parse --parseopt -- "$@" || echo exit $?)"
 ------------
 
 SQ-QUOTE