hashmap: adjust documentation to reflect reality

hashmap: adjust documentation to reflect reality

The hashmap API is just complicated enough that even at least one
long-time Git contributor has to look up how to use it every time he
finds a new use case. When that happens, it is really useful if the
provided example code is correct...

While at it, "fix a memory leak", avoid statements before variable
declarations, fix a const -> no-const cast, several %l specifiers (which
want to be %ld), avoid using an undefined constant, call scanf()
correctly, use FLEX_ALLOC_STR() where appropriate, and adjust the style
here and there.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>