rev-parse: be more careful with munging arguments

rev-parse: be more careful with munging arguments

When rev-parse looks at whether an argument like "foo..bar" or
"foobar^@" is a difference or parent-shorthand, it internally
munges the arguments so that it can pass the individual rev
arguments to get_sha1(). However, we do not consistently un-munge
the result.

For cases where we do not match (e.g., "doesnotexist..HEAD"), we
would then want to try to treat the argument as a filename.
try_difference gets() this right, and always unmunges in this case.
However, try_parent_shorthand() never unmunges, leading to incorrect
error messages, or even incorrect results:

  $ git rev-parse foobar^@
  foobar
  fatal: ambiguous argument 'foobar': unknown revision or path not in the working tree.
  Use '--' to separate paths from revisions, like this:
  'git <command> [<revision>...] -- [<file>...]'

  $ >foobar
  $ git rev-parse foobar^@
  foobar

For cases where we do match, neither function unmunges. This does
not currently matter, since we are done with the argument. However,
a future patch will do further processing, and this prepares for
it. In addition, it's simply a confusing interface for some cases to
modify the const argument, and others not to.

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>