magit.el: add "Compatibility/Emacs" (sub)section headers

Signed-off-by: Pieter Praet <pieter@praet.org>
magit.el: restore code provenance statement

Commit 445e6e8d moved the Emacs compatibility functions from
"magit-compat.el" into "magit.el" but dropped the headers,
thereby losing the code provenance statement.

Signed-off-by: Pieter Praet <pieter@praet.org>
magit.el: restore compatibility statement

Commit 445e6e8d moved the Emacs compatibility functions from
"magit-compat.el" into "magit.el" but dropped the headers,
thereby losing the compatibility statement.

Signed-off-by: Pieter Praet <pieter@praet.org>
magit.el: clean up compatibility statement

Signed-off-by: Pieter Praet <pieter@praet.org>
magit.el: fix Emacs compat function versions

Specify in which commits the functions were added to Emacs, and correct
the version numbers for `string-prefix-p' and `string-match-p'.

Note that we try to ensure compatibility with Emacs 23.2 and up, so the
compatibility functions for `string-prefix-p' (added in 23.2 instead of
23.3) and `string-match-p' (added in Emacs 23.1 instead of 23.3) are
nothing but clutter, meaning we should probably sacrifice them to Lord
Lambda so we can recycle the parens.

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #927 from praet/compat

Emacs compatibility stuff