write-or-die: make GIT_FLUSH a Boolean environment variable

Among Git's environment variables, the ones marked as "Boolean"
accept values in a way similar to Boolean configuration variables,
i.e. values like 'yes', 'on', 'true' and positive numbers are
taken as "on" and values like 'no', 'off', 'false' are taken as
"off".

GIT_FLUSH can be used to force Git to use non-buffered I/O when
writing to stdout. It can only accept two values, '1' which causes
Git to flush more often and '0' which makes all output buffered.
Make GIT_FLUSH accept more values besides '0' and '1' by turning it
into a Boolean environment variable, modifying the required logic.
Update the related documentation.

Signed-off-by: Chandra Pratap <chandrapratap3519@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git.txt

@@ -724,13 +724,12 @@ for further details.
  waiting for someone with sufficient permissions to fix it.
 
 `GIT_FLUSH`::
-// NEEDSWORK: make it into a usual Boolean environment variable
- If this environment variable is set to "1", then commands such
+ If this Boolean environment variable is set to true, then commands such
  as 'git blame' (in incremental mode), 'git rev-list', 'git log',
  'git check-attr' and 'git check-ignore' will
  force a flush of the output stream after each record have been
  flushed. If this
- variable is set to "0", the output of these commands will be done
+ variable is set to false, the output of these commands will be done
  using completely buffered I/O. If this environment variable is
  not set, Git will choose buffered or record-oriented flushing
  based on whether stdout appears to be redirected to a file or not.

write-or-die.c

@@ -19,20 +19,17 @@
 void maybe_flush_or_die(FILE *f, const char *desc)
 {
  static int skip_stdout_flush = -1;
- struct stat st;
- char *cp;
 
  if (f == stdout) {
  if (skip_stdout_flush < 0) {
- /* NEEDSWORK: make this a normal Boolean */
- cp = getenv("GIT_FLUSH");
- if (cp)
- skip_stdout_flush = (atoi(cp) == 0);
- else if ((fstat(fileno(stdout), &st) == 0) &&
- S_ISREG(st.st_mode))
- skip_stdout_flush = 1;
- else
- skip_stdout_flush = 0;
+ skip_stdout_flush = git_env_bool("GIT_FLUSH", -1);
+ if (skip_stdout_flush < 0) {
+ struct stat st;
+ if (fstat(fileno(stdout), &st))
+ skip_stdout_flush = 0;
+ else
+ skip_stdout_flush = S_ISREG(st.st_mode);
+ }
  }
  if (skip_stdout_flush && !ferror(f))
  return;