use `git -c g.o=v cmd' only when git supports it

Fixes #662.
Merge branch 'maint'
improve installation instructions and other docs
Merge branch 'feature/readme' into maint
Merge branch 'maint'

Conflicts:
	README.md
Make `magit-guess-branch' have git grep the reflog for at most one branch name.

This uses "git log --walk-reflogs --grep-reflog PATTERN, which is
supported in git >= 1.8.0.

Conflicts:
	magit.el
Merge branch 'feature/quick-reflog'
magit-refresh-log-buffer: fix section heading

Don't claim commits are in HEAD when they are not.
Ensure that the magit-insert-status-line second argument is a known string

This second argument being is passed as first argument of format, and
should contain %d and other format parameter only when appropriate.

Close #667