define magit-single-file-log command and bind it in logging popup
fixed some style issues
added dedicated history variable for magit-stash
defined dedicated washer for unpulled and unpushed commits
added tests for unpushed commits
remove hard-coded value

In the future, this constant should disappear anyway.
don't check for exact buffer name

This is unnecessarily retrictive. check for compatible major mode instead.
fix issue 628

magit-log-author-date-overlay wasn't buffer-local
rename internal functions
Merge branch 'sergv/master'
Merge branch 'issue-628'