write_file(): drop "fatal" parameter

All callers except three passed 1 for the "fatal" parameter to ask
this function to die upon error, but to a casual reader of the code,
it was not all obvious what that 1 meant.  Instead, split the
function into two based on a common write_file_v() that takes the
flag, introduce write_file_gently() as a new way to attempt creating
a file without dying on error, and make three callers to call it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -1539,8 +1539,9 @@ static inline ssize_t write_str_in_full(int fd, const char *str)
 {
 	return write_in_full(fd, str, strlen(str));
 }
-__attribute__((format (printf, 3, 4)))
-extern int write_file(const char *path, int fatal, const char *fmt, ...);
+
+extern int write_file(const char *path, const char *fmt, ...);
+extern int write_file_gently(const char *path, const char *fmt, ...);
 
 /* pager.c */
 extern void setup_pager(void);