strbuf API additions and enhancements.

strbuf API additions and enhancements.

Add strbuf_remove, change strbuf_insert:
  As both are special cases of strbuf_splice, implement them as such.
  gcc is able to do the math and generate almost optimal code this way.

Add strbuf_swap:
  Exchange the values of its arguments.
  Use it in fast-import.c

Also fix spacing issues in strbuf.h

Signed-off-by: Pierre Habouzit <madcoder@debian.org>