mergetool: Use args as pathspec to unmerged files

mergetool: Use args as pathspec to unmerged files

Mergetool now treats its path arguments as a pathspec (like other git
subcommands), restricting action to the given files and directories.
Files matching the pathspec are filtered so mergetool only acts on
unmerged paths; previously it would assume each path argument was in an
unresolved state, and get confused when it couldn't check out their
other stages.

Running "git mergetool subdir" will prompt to resolve all conflicted
blobs under subdir.

Signed-off-by: Jonathon Mah <me@JonathonMah.com>
Acked-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>