magit-blame-format-time-string: bugfix
magit-blame-{header,sha1}: fix faces

These faces inherited from faces which no longer exist.
Fix `magit-blame-sha1' by inheriting from `magit-hash',
and `magit-blame-header' by defining it from scratch.
rename magit-blame-{header => heading, sha1 => hash}

for consistency with `magit-{section-heading,hash}'.
magit-blame-file-on: turn off incompatible fci-mode

Because the current magit-blame.el will soon be replaced with
a new implementation, don't bother turning `fci-mode' on again
when `magit-blame-mode' is turned off.