config: allow tweaking whitespace between value and comment

Extending the previous step, this allows the whitespace placed after the value before the "# comment message" to be tweaked by tweaking the preprocessing rule to: * If the given comment string begins with one or more whitespace characters followed by '#', it is passed intact. * If the given comment string begins with '#', a Space is prepended. * Otherwise, " # " (Space, '#', Space) is prefixed. * A string with LF in it cannot be used as a comment string. Unlike the previous step, which unconditionally added a space after the value before writing the "# comment string", because the above preprocessing already gives a whitespace before the '#', the resulting string is written immediately after copying the value. And the sanity checking rule becomes * comment string after the above massaging that comes into git_config_set_multivar_in_file_gently() must - begin with zero or more whitespace characters followed by '#'. - not have a LF in it. I personally think this is over-engineered, but since I thought things through anyway, here it is in the patch form. The logic to tweak end-user supplied comment string is encapsulated in a new helper function, git_config_prepare_comment_string(), so if new front-end callers would want to use the same massaging rules, it is easily reused. Unfortunately I do not think of a way to tweak the preprocessing rules further to optionally allow having no blank after the value, i.e. to produce [section] variable = value#comment (which is a valid way to say section.variable=value, by the way) without sacrificing the ergonomics for the more usual case, so this time I really stop here. Signed-off-by: Junio C Hamano <gitster@pobox.com>