add-patch: handle splitting hunks with diff.suppressBlankEmpty

When "add -p" parses diffs, it looks for context lines starting with a single space. But when diff.suppressBlankEmpty is in effect, an empty context line will omit the space, giving us a true empty line. This confuses the parser, which is unable to split based on such a line. It's tempting to say that we should just make sure that we generate a diff without that option. However, although we do not parse hunks that the user has manually edited with parse_diff() we do allow the user to split such hunks. As POSIX calls the decision of whether to print the space here "implementation-defined" we need to handle edited hunks where empty context lines omit the space. So let's handle both cases: a context line either starts with a space or consists of a totally empty line by normalizing the first character to a space when we parse them. Normalizing the first character rather than changing the code to check for a space or newline will hopefully future proof against introducing similar bugs if the code is changed. Reported-by: Ilya Tumaykin <itumaykin@gmail.com> Helped-by: Jeff King <peff@peff.net> Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk> Signed-off-by: Junio C Hamano <gitster@pobox.com>