Alter rev-parse syntax to support msys git

On windows, msys and cygwin versions of git need braces to be escaped when they 
are invoked via subprocess.  (This appears to be is a side effect of the machinery 
that converts windows paths to posix-like paths for the executable).

If they are not escaped, then "rev^{commit]" becomes a meaningless "rev^commit" 
to an msys git, resulting in an error.

In contrast, we do not want to escape the braces if passing "rev^{commit}" to any
other form of git.  We can avoid having to decide whether escaping is needed by
using the alternative notation "^0".

poetry/vcs/git.py

@@ -229,9 +229,15 @@ def rev_parse(self, rev, folder=None):  # type: (...) -> str
                 folder.as_posix(),
             ]
 
-        # We need "^{commit}" to ensure that the commit SHA of the commit the
-        # tag points to is returned, even in the case of annotated tags.
-        args += ["rev-parse", rev + "^{commit}"]
+        # We need "^0" (an alternative to "^{commit}") to ensure that the
+        # commit SHA of the commit the tag points to is returned, even in
+        # the case of annotated tags.
+        #
+        # We deliberately avoid the "^{commit}" syntax itself as on some 
+        # platforms (cygwin/msys to be specific), the braces are interpreted
+        # as special characters and would require escaping, while on others
+        # they should not be escaped.
+        args += ["rev-parse", rev + "^0"]
 
         return self.run(*args)