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 wouuld 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^@
  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^@

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 <p...@peff.net>
As an aside, this is one of those places where C's string functions do
gross things with const. We modify a const argument to the function
without any casts or warnings, because the return value from strstr
implicitly drops the const. Time for us all to move to C++ with
templated strstr, right?

 builtin/rev-parse.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/builtin/rev-parse.c b/builtin/rev-parse.c
index bb694ab..a60fcd3 100644
--- a/builtin/rev-parse.c
+++ b/builtin/rev-parse.c
@@ -286,6 +286,7 @@ static int try_difference(const char *arg)
                                exclude = n;
+               *dotdot = '.';
                return 1;
        *dotdot = '.';
@@ -309,8 +310,10 @@ static int try_parent_shorthands(const char *arg)
                return 0;
        *dotdot = 0;
-       if (get_sha1_committish(arg, sha1))
+       if (get_sha1_committish(arg, sha1)) {
+               *dotdot = '^';
                return 0;
+       }
        if (!parents_only)
                show_rev(NORMAL, sha1, arg);
@@ -319,6 +322,7 @@ static int try_parent_shorthands(const char *arg)
                show_rev(parents_only ? NORMAL : REVERSED,
                                parents->item->object.sha1, arg);
+       *dotdot = '^';
        return 1;

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