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>

Documentation/doc-diff

@@ -77,7 +77,7 @@ generate_render_makefile () {
 		printf '%s: %s\n' "$dst" "$src"
 		printf '\t@echo >&2 "  RENDER $(notdir $@)" && \\\n'
 		printf '\tmkdir -p $(dir $@) && \\\n'
-		printf '\tMANWIDTH=80 man -l $< >$@+ && \\\n'
+		printf '\tMANWIDTH=80 man $< >$@+ && \\\n'
 		printf '\tmv $@+ $@\n'
 	done
 }