Git only writes the space after a marker when there is a label to write
after it, and the label can be empty: `git checkout -m` with the diff3
conflict style, for instance, has no name for the common ancestor, so it
writes a bare "|||||||" line.
Git doesn't always write conflict markers of seven characters: the
conflict-marker-size gitattribute overrides that per file, and it is set
for good reasons — for file types whose regular content tends to contain
marker-looking lines, such as documentation about merging, or test
scripts. We hard-code seven characters everywhere we look for markers,
so none of that works.
Prepare for honoring the attribute by threading the marker size through
everything that recognizes a marker, carried on the file model. Nothing
fills it in yet, so we still use git's default size of seven everywhere,
and matching is unchanged: a marker consists of exactly that many marker
characters, and all but the "=======" one are followed by a space and a
label.
