Merge branch 'da/mergetool-temporary-filename'

Merge branch 'da/mergetool-temporary-filename'

Tweak the names of the three throw-away files "git mergetool" comes
up with to feed the merge tool backend, so that a file with a
single dot in its name in the original (e.g. "hello.c") will have
only one dot in these variants (e.g. "hello_BASE_4321.c").

* da/mergetool-temporary-filename:
  mergetool: use more conservative temporary filenames