Differentiate between first and subsequent entries for the same function

[krlmlr]

This helps understand if we're looking at debug messages from different or from the same function.

For f() printing one message and then calling g() which prints 3 messages, this gives:

Before:

f
+-g1
+-g2
+-g3

After:

f
+-g1
 -g2
 -g3

[gaborcsardi]

Super cool! Thanks a lot!