Fix encoding issues in gitmerge.el, make it work on MS-Windows

Fix encoding issues in gitmerge.el, make it work on MS-Windows

* admin/gitmerge.el (gitmerge-emacs-version, gitmerge-show-log)
(gitmerge-show-diff, gitmerge-show-files, gitmerge-missing)
(gitmerge-setup-log-buffer, gitmerge-resolve)
(gitmerge-commit-message, gitmerge-apply)
(gitmerge-resolve-unmerged, gitmerge-repo-clean)
(gitmerge-commit): Bind coding-system-for-read/write to the
appropriate vc-git values.
(gitmerge-apply) [windows-nt]: Pass long merge log messages via a
temporary file.
(gitmerge): Clarify the description of the "R" flag.