read-tree: explicitly disallow prefixes with a leading '/'

Exit with an error if a prefix provided to `git read-tree --prefix` begins with '/'. In most cases, prefixes like this result in an "invalid path" error; however, the repository root would be interpreted as valid when specified as '--prefix=/'. This is due to leniency around trailing directory separators on prefixes (e.g., allowing both '--prefix=my-dir' and '--prefix=my-dir/') - the '/' in the prefix is actually the *trailing* slash, although it could be misinterpreted as a *leading* slash. To remove the confusing repo root-as-'/' case and make it clear that prefixes should not begin with '/', exit with an error if the first character of the provided prefix is '/'. Helped-by: Elijah Newren <newren@gmail.com> Signed-off-by: Victoria Dye <vdye@github.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>