merge database may lose entries on fingerprint mismatch

[jhnc]

Given:

db1 = { img1 => fp1a, img2 => fp2 }
db2 = { img1 => fp1b, img2 => fp2 }

it looks like:

findimagedupes -f db1 -f db2 -M db3 → db3 = { img1 => fp1c, img2 => fp2 }

and

findimagedupes -f db1 -f db2 -M db3 -- img3 → db3 = { img2 => fp2, img3 => fp3 }

However, the latter should give: db3 = { img1 => fp1c, img2 => fp2, img3 => fp3 }

The @regen array is ignored when explicit filelist is provided on the commandline.

[jhnc]

Caused by 4e06a13, which was supposed to prevent files queued for re-processing (d68aad5) being treated as -a arguments