Merge branch 'mr/bisect-in-c-4'

Rewrite of "git bisect" in C continues.

* mr/bisect-in-c-4:
  bisect--helper: retire `--bisect-next-check` subcommand
  bisect--helper: reimplement `bisect_run` shell function in C
  bisect--helper: reimplement `bisect_visualize()` shell function in C
  run-command: make `exists_in_PATH()` non-static
  t6030-bisect-porcelain: add test for bisect visualize
  t6030-bisect-porcelain: add tests to control bisect run exit cases

run-command.h

@@ -190,6 +190,18 @@ void child_process_clear(struct child_process *);
 
 int is_executable(const char *name);
 
+/**
+ * Check if the command exists on $PATH. This emulates the path search that
+ * execvp would perform, without actually executing the command so it
+ * can be used before fork() to prepare to run a command using
+ * execve() or after execvp() to diagnose why it failed.
+ *
+ * The caller should ensure that command contains no directory separators.
+ *
+ * Returns 1 if it is found in $PATH or 0 if the command could not be found.
+ */
+int exists_in_PATH(const char *command);
+
 /**
  * Start a sub-process. Takes a pointer to a `struct child_process`
  * that specifies the details and returns pipe FDs (if requested).