accept more version names in file-name-sans-versions

[Dan Nicolaescu]

For a file file.c managed by git doing
C-u C-x v = master RET

will create the buffer file.c.~master~ with the major-mode
fundamental-mode not c-mode.

The reason is that file-name-sans-versions only looks for revision
names matching ".~[0-9.]+~", so .~master~ does not match. 

Any reason not to add a-z to that regexp?