blame: tighten command line parser

blame: tighten command line parser

The command line parser of "git blame" is prepared to take an
ancient odd argument order "blame <path> <rev>" in addition to the
usual "blame [<rev>] <path>".  It has at least two negative
ramifications:

 - In order to tell these two apart, it checks if the last command
   line argument names a path in the working tree, using
   file_exists().  However, "blame <rev> <path>" is a request to
   explain each and every line in the contents of <path> stored in
   revision <rev> and does not need to have a working tree version
   of the file.  A check with file_exists() is simply wrong.

 - To coerce that mistaken file_exists() check to work, the code
   calls setup_work_tree() before doing so, because the path it has
   is relative to the top-level of the project tree.  However,
   "blame <rev> <path>" MUST be usable even in a bare repository,
   and there is no reason for letting setup_work_tree() complain
   and die with "This operation must be run in a work tree".

To correct the former, switch to check if the last token is a
revision (and if so, parse arguments using "blame <path> <rev>"
rule).  Correct the latter by getting rid of setup_work_tree() and
file_exists() check--the only case the call to this function matters
is when we are running "blame <path>" (i.e. no starting revision and
asking to blame the working tree file at <path>, digging through the
HEAD revision), but there is a call in setup_scoreboard() just
before it calls fake_working_tree_commit().

Signed-off-by: Junio C Hamano <gitster@pobox.com>