Fix return value of `magit-ref-exists-p'

It causes new branch cannot be created in branch manager.
Merge pull request #871 from aethanyc/fix-return-value

Fix return value of `magit-ref-exists-p'
remove requirement on a graphical environment

As Donald has pointed out and I and others have confirmed
we can use emacsclient without a graphical environment.
magit-tag: ensure use of emacsclient when using --sign

Closes #863.
commit: add -v for verbose option

Add a -v option which allows you to invoke git-commit with --verbose. I
always do this because inevitably I'll forget some of the finer points
of the patch I'm about to commit, and having it all in the same buffer
instead of having to look at "git diff --staged" in another window is
great.

Now when you press "c" the window that's brought up looks like this:

    Switches
     -r: Replace the tip of current branch (--amend)         -a: Stage all modified and deleted files (--all)
     -e: Allow empty commit (--allow-empty)                  -v: Show diff of changes to be committed (--verbose)
     -n: Bypass git hooks (--no-verify)                      -s: Add Signed-off-by line (--signoff)
     -S: Sign using gpg (--gpg-sign)
    Actions
     c: Commit

And e.g. for this change I'm about to commit the buffer opens with:

    # Please enter the commit message for your changes. Lines starting
    # with '#' will be ignored, and an empty message aborts the commit.
    # On branch master
    # Changes to be committed:
    #   (use "git reset HEAD <file>..." to unstage)
    #
    #	modified:   magit-key-mode.el
    #
    diff --git a/magit-key-mode.el b/magit-key-mode.el
    index a7d9a8c..4fd5510 100644
    --- a/magit-key-mode.el
    +++ b/magit-key-mode.el
    @@ -192,6 +192,7 @@
           ("-r" "Replace the tip of current branch" "--amend")
           ("-a" "Stage all modified and deleted files" "--all")
           ("-e" "Allow empty commit" "--allow-empty")
    +      ("-v" "Show diff of changes to be committed" "--verbose")
           ("-n" "Bypass git hooks" "--no-verify")
           ("-s" "Add Signed-off-by line" "--signoff")
           ("-S" "Sign using gpg" "--gpg-sign")))
magit-rebase-step: assert emacsclient when --continue_ing

Fix issue #873.
fix c4f4c096becaca65bf222122e77409cd61fedf9a
magit-commit-internal: remove EDITMSG after commit

Remove file .git/{TAG,COMMIT}_EDITMSG after making
a commit/tag using the emacsclient-less fallback.

Closes #797.
magit-commit-internal: set git-commit-previous-winconf
magit-commit-internal: refresh magit buffers with fallback too
magit-commit-add-log: reduce nesting
magit-commit-add-log: initiate commit if necessary

Close #829.
Merge pull request #872 from avar/add-v-option-to-commit

commit: add -v for verbose option
fix 6e4d21b6ef7c7dbc483cda5f17a541bb954224b5

Reported-by: Johan Andersson <johan.rejeep@gmail.com>
fix two bugs in magit-commit

* Make sure we don't set `magit-custom-options' global value.
* Respect the prefix argument (AMENDP) when there are staged changes.
make commit at point the default input value for magit-show-commit