Accept 'inline' file data in fast-import commit structure.

Accept 'inline' file data in fast-import commit structure.

Its very annoying to need to specify the file content ahead of a
commit and use marks to connect the individual blobs to the commit's
file modification entry, especially if the frontend can't/won't
generate the blob SHA1s itself.  Instead it would much easier to
use if we can accept the blob data at the same time as we receive
each file_change line.

Now fast-import accepts 'inline' instead of a mark idnum or blob
SHA1 within the 'M' type file_change command.  If an inline is
detected the very next line must be a 'data n' command, supplying
the file data.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>