magit-insert-untracked-files: respect git variable

In `magit-insert-untracked-files' respect the git variable
`status.showUntrackedFiles'.  This fixes a bug (we didn't respect
a value of "no") and works around a bug in git (when `-u' is used
without a mode suffix "all" is used, not "normal" is the manpages
claim).
use re-search-forward instead of alias search-forward-regexp
magit-visit-item: move definition
magit-visit-item: cosmetics
magit-visit-item: don't needlessly use call-interactively
collect all ediff wrappers in one place
magit-interactive-resolve: absorb magit-interactive-resolve-item
magit-ediff-buffers{,3}: new functions

Add `magit-ediff-buffers' and `magit-ediff-buffers3',
replacing `magit-ediff*'.
magit-make-commit-button: bugfix
Revert "magit-make-commit-button: bugfix"

This reverts commit 10fd6ae187f817c59561cb67aab1bd1bfeb82068.
magit-make-commit-button: properly fix bug
magit-insert-status-head-line: insert commit section
magit-wash-diff-section: hide body of deleted file sections
magit-key-mode-popup-logging: add -L option

Also see #909.
magit-insert-status-head-line: verify HEAD exists
fix off-by-one error in magit-wash-commit
fix section's title being used where info (file) should have been

This fixes 2 bugs:
- "Can't visit deleted file: diff --git a/magit.el b/magit.el"
- Empty buffers when calling magit-ediff on a hunk in magit-commit-mode

"info" slot should probably be renamed into "file"
Merge branch 'pr/fix-ediff-and-visit-item'
magit-describe-item: use marker-position
magit-describe-item: show CONTENT-BEGINNING

For header-only sections only show the values of the BEGINNING and END
section slots.  Note that due to a bug in `magit-with-section' the value
of the CONTENT-BEGINNING of header-only sections is bogus, so we have to
detect that explicitly for the time being.