blame: enable funcname blaming with userdiff driver

blame: enable funcname blaming with userdiff driver

In blame.c::cmd_blame, we send the 'path' field of the 'sb' 'struct
blame_scoreboard' as the 'path' argument to
'line-range.c::parse_range_arg', but 'sb.path' is not set yet; it's set
to the local variable 'path' a few lines later at line 1137.

This 'path' argument is only used in 'parse_range_arg' if we are blaming
a funcname, i.e. `git blame -L :<funcname> <path>`, and in that case it
is sent to 'parse_range_funcname', where it is used to determine if a
userdiff driver should be used for said <path> to match the given
funcname.

Since 'path' is yet unset, the userdiff driver is never used, so we fall
back to the default funcname regex, which is usually not appropriate for
paths that are set to use a specific userdiff driver, and thus either we
match some unrelated lines, or we die with

    fatal: -L parameter '<funcname>' starting at line 1: no match

This has been the case ever since `git blame` learned to blame a
funcname in 13b8f68c1f (log -L: :pattern:file syntax to find by
funcname, 2013-03-28).

Enable funcname blaming for paths using specific userdiff drivers by
initializing 'sb.path' earlier in 'cmd_blame', when some of its other
fields are initialized, so that it is set when passed to
'parse_range_arg'.

Add a regression test in 'annotate-tests.sh', which is sourced in
t8001-annotate.sh and t8002-blame.sh, leveraging an existing file used
to test the userdiff patterns in t4018-diff-funcname.

Also, use 'sb.path' instead of 'path' when constructing the error
message at line 1114, for consistency.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>