fix(tools/checkpatch): fix CHECKPATCH_RUN=1 for multiple files in change

The content of the source_files variable needs to be a list (items
separated by ';') and needs to be passed without quotes to 'git diff'
in the execute_process call, or else the list is treated as a single
filename.

Change-Id: I783347f1d2e6863d01329bf3e22655bd053b34e1
Signed-off-by: Mate Toth-Pal <mate.toth-pal@arm.com>