trace: avoid unnecessary quoting

Trace output which contains arbitrary strings (e.g., the
arguments to commands which we are running) is always passed
through sq_quote_buf(). That function always adds
single-quotes, even if the output consists of vanilla
characters. This can make the output a bit hard to read.

Let's avoid the quoting if there are no characters which a
shell would interpret. Trace output doesn't necessarily need
to be shell-compatible, but:

  - the shell language is a good ballpark for what humans
    consider readable (well, humans versed in command line
    tools)

  - the run_command bits can be cut-and-pasted to a shell,
    and we'll keep that property

  - it covers any cases which would make the output
    visually ambiguous (e.g., embedded whitespace or quotes)

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

quote.h

@@ -33,6 +33,14 @@ extern void sq_quote_buf(struct strbuf *, const char *src);
 extern void sq_quote_argv(struct strbuf *, const char **argv);
 extern void sq_quotef(struct strbuf *, const char *fmt, ...);
 
+/*
+ * These match their non-pretty variants, except that they avoid
+ * quoting when there are no exotic characters. These should only be used for
+ * human-readable output, as sq_dequote() is not smart enough to dequote it.
+ */
+void sq_quote_buf_pretty(struct strbuf *, const char *src);
+void sq_quote_argv_pretty(struct strbuf *, const char **argv);
+
 /* This unwraps what sq_quote() produces in place, but returns
  * NULL if the input does not look like what sq_quote would have
  * produced.