diff: factor out --follow pathspec check

In --follow mode, we require exactly one pathspec. We check this
condition in two places:

  - in diff_setup_done(), we complain if --follow is used with an
    inapropriate pathspec

  - in git-log's revision "tweak" function, we enable log.follow only if
    the pathspec allows it

The duplication isn't a big deal right now, since the logic is so
simple. But in preparation for it becoming more complex, let's pull it
into a shared function.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.h

@@ -542,6 +542,13 @@ void repo_diff_setup(struct repository *, struct diff_options *);
 struct option *add_diff_options(const struct option *, struct diff_options *);
 int diff_opt_parse(struct diff_options *, const char **, int, const char *);
 void diff_setup_done(struct diff_options *);
+
+/*
+ * Returns true if the pathspec can work with --follow mode. If die_on_error is
+ * set, die() with a specific error message rather than returning false.
+ */
+int diff_check_follow_pathspec(struct pathspec *ps, int die_on_error);
+
 int git_config_rename(const char *var, const char *value);
 
 #define DIFF_DETECT_RENAME	1