fix: incorrect regular expression matching diffcmd

[jingfelix]

current diffcmd_header: ^diff.* (.+) (.+)$ fixed diffcmd_header: ^diff .* (.+) (.+)$

Using the old diffcmd_header may result in lines starting with words like "different" being incorrectly matched as a diff header. Fixing this by adding a blank space.

example: this commit in Linux kernel