magit-load-config-extensions: move definition
magit-init: move definition
magit-save-some-buffers-topdir: rename variable

Rename `magit-default-directory' to `magit-save-some-buffers-topdir'.
Considering that this variable is only used (abused) for one rather
specific thing, the old name was way to generic.
magit-save-some-buffers{*}: move definitions
prepare major rearrangement
magit-set, magit-get{,-{*}}: move definitions
magit-{,{*}-}completing-read: move definitions
populate "Revision Completion" section
populate "Miscellaneous Completion" section
populate "Repository Paths" section
populate "Repository Predicates" section
populate "Diff Predicates" section
populate "Revisions and References" section
Merge branch 'n/rearrange-1' into next