Pass the log message by strbuf instead of as a C-style string and use
fwrite instead of printf to write it to fast-import so embedded '\0'
bytes can be preserved.
Currently "git log" doesn't show the embedded NULs but "git cat-file
commit" can.
While at it, stop including system headers from repo_tree.h. git
source files need to include git-compat-util.h (or cache.h or
builtin.h) sooner to ensure the appropriate feature test macros are
defined.
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>