Limit the display of diff output to 16 KB to not freeze the UI while rendering

This should workaround the long-standing bug that the commit dialog freezes Geany
when the diff output is very large. Now, if the output exceeds 16 KB, a short message
is displayed instead suggesting to open the diff for review in Geany directly.
Merge pull request #105 from eht16/big_diff_commit_fix

Limit the display of diff output to 16 KB to not freeze the UI while ren...