[git] Push dialog: for new branch show commits which are not pushed yet

Show all commits that are in the given branch,
but not in any remote branches in the given remote:
`git log new_branch --not --remotes=origin`
This fixes IDEA-83227.
[git] Pull: remove fetch+merge hack used for JGit

JGit is no more used for HTTP operations.

This fixes IDEA-101902: git pull will be called only for the
chosen remote.
[git] IDEA-101668 Fix Git log for detached HEAD

`git log --branches --tags --remotes` doesn't count the current HEAD
if it is detached. To fix it, show the log from the HEAD:
`git log HEAD --branches --tags --remotes`