doc-diff: fix non-portable 'man' invocation

doc-diff invokes 'man' with the -l option to force "local" mode, however, neither MacOS nor FreeBSD recognize this option. On those platforms, if the argument to 'man' contains a slash, it is automatically interpreted as a file specification, so a "local"-like mode is not needed. And, it turns out, 'man' which does support -l falls back to enabling -l automatically if it can't otherwise find a manual entry corresponding to the argument. Since doc-diff always passes an absolute path of the nroff source file to 'man', the -l option kicks in anyhow, despite not being specified explicitly. Therefore, make the invocation portable to the various platforms by simply dropping -l. Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>