checkpatch: fix the usage of capture group ( ... )

The usage of "capture group (...)" in the immediate condition after `&&`
results in `$1` being uninitialized.  This issues a warning "Use of
uninitialized value $1 in regexp compilation at ./scripts/checkpatch.pl
line 2638".

I noticed this bug while running checkpatch on the set of commits from
v5.7 to v5.8-rc1 of the kernel on the commits with a diff content in
their commit message.

This bug was introduced in the script by commit e518e9a5
("checkpatch: emit an error when there's a diff in a changelog").  It
has been in the script since then.

The author intended to store the match made by capture group in variable
`$1`.  This should have contained the name of the file as `[\w/]+`
matched.  However, this couldn't be accomplished due to usage of capture
group and `$1` in the same regular expression.

Fix this by placing the capture group in the condition before `&&`.
Thus, `$1` can be initialized to the text that capture group matches
thereby setting it to the desired and required value.

Fixes: e518e9a5 ("checkpatch: emit an error when there's a diff in a changelog")
Signed-off-by: Mrinal Pandey <mrinalmni@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Cc: Joe Perches <joe@perches.com>
Link: https://lkml.kernel.org/r/20200714032352.f476hanaj2dlmiot@mrinalpandey


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>