vim: skip trailing slash for g:notmuch_compose_temp_file_dir

vim: skip trailing slash for g:notmuch_compose_temp_file_dir

With the trailing slash I get

	Error detected while processing function <SNR>10_NM_new_mail..<SNR>10_NM_cmd_compose..<SNR>10_NM_newComposeBuffer..<SNR>10_NM_newFileBuffer:
	line    3:
	E739: Cannot create directory: /home/ukleinek/.notmuch/compose/

when hitting 'm' to compose a new mail. strace shows:

	stat("/home/ukleinek/.notmuch/compose/", 0x7fffee314a10) = -1 ENOENT (No such file or directory)
	stat("/home/ukleinek/.notmuch/compose/", 0x7fffee314e30) = -1 ENOENT (No such file or directory)
	stat("/home/ukleinek/.notmuch/compose", 0x7fffee315270) = -1 ENOENT (No such file or directory)
	stat("/home/ukleinek/.notmuch", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
	mkdir("/home/ukleinek/.notmuch/compose", 0755) = 0
	mkdir("/home/ukleinek/.notmuch/compose/", 0755) = -1 EEXIST (File exists)

so it seems vim's mkdir() isn't able to handle a trailing slash.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>