object-name: fix resolution of object names containing curly braces

object-name: fix resolution of object names containing curly braces

Given a branch name of 'foo{bar', commands like

    git cat-file -p foo{bar:README.md

should succeed (assuming that branch had a README.md file, of course).
However, the change in cce91a2caef9 (Change 'master@noon' syntax to
'master@{noon}'., 2006-05-19) presumed that curly braces would always
come after an '@' or '^' and be paired, causing e.g. 'foo{bar:README.md'
to entirely miss the ':' and assume there's no object being referenced.
In short, git would report:

    fatal: Not a valid object name foo{bar:README.md

Change the parsing to only make the assumption of paired curly braces
immediately after either a '@' or '^' character appears.

Add tests for this, as well as for a few other test cases that initial
versions of this patch broke:
  * 'foo@@{...}'
  * 'foo^{/${SEARCH_TEXT_WITH_COLON}}:${PATH}'

Note that we'd prefer not duplicating the special logic for "@^" characters
here, because if get_oid_basic() or interpret_nth_prior_checkout() or
get_oid_basic() or similar gain extra methods of using curly braces,
then the logic in get_oid_with_context_1() would need to be updated as
well.  But it's not clear how to refactor all of these to have a simple
common callpoint with the specialized logic.

Reported-by: Gabriel Amaral <gabriel-amaral@github.com>
Helped-by: Michael Haggerty <mhagger@github.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>