subtree: use 'git rev-parse --verify [--quiet]' for better error messages

There are three occurences of 'git rev-parse <rev>' in 'git-subtree.sh' where the command expects a revision and the script dies or exits if the revision can't be found. In that case, the error message from 'git rev-parse' is: $ git rev-parse <bad rev> <bad rev> fatal: ambiguous argument '<bad rev>': unknown revision or path not in the working tree. Use '--' to separate paths from revisions, like this: 'git <command> [<revision>...] -- [<file>...]' This is a little confusing to the user, since this error message is outputed by 'git subtree'. At these points in the script, we know that we are looking for a single revision, so be explicit by using '--verify', resulting in a little better error message: $ git rev-parse --verify <bad rev> fatal: Needed a single revision In the two occurences where we 'die' if 'git rev-parse' fails, 'git subtree' outputs "could not rev-parse split hash $b from commit $sq", so we actually do not need the supplementary error message from 'git rev-parse'; add '--quiet' to silence it. In the third occurence, we 'exit', so keep the error message from 'git rev-parse'. Note that this messsage is still suboptimal since it can be understood to mean that 'git rev-parse' did not receive a single revision as argument, which is not the case here: the command did receive a single revision, but the revision is not resolvable to an available object. The alternative would be to use '--' after the revision, as suggested by the first error message, resulting in a clearer error message: $ git rev-parse <bad rev> -- fatal: bad revision '<bad rev>' Unfortunately we can't use that syntax because in the more common case of the revision resolving to a known object, the command outputs the object's hash, a newline, and the dashdash, which breaks the 'git subtree' script. Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>