Make format_pieces recognize the \e escape sequence

I noticed that the printf command did not recognize the \e escape
sequence, used amongst other things to use colors:

  (gdb) printf "This is \e[32mgreen\e[m!\n"
  Unrecognized escape character \e in format string.

This patch makes format_pieces recognize it, which makes that command
print the expected result in glorious color.

I wrote a really simple unit test for format_pieces.
format_pieces::operator[] is unused so I removed it.  I added
format_piece::operator==, which is needed to compare vectors of
format_piece.

gdb/ChangeLog:

	PR cli/14975
	* Makefile.in (SUBDIR_UNITTESTS_SRCS): Add
	unittests/format_pieces-selftests.c.
	* common/format.h (format_piece) <operator==>: New.
	(format_pieces) <operator[]>: Remove.
	* common/format.c (format_pieces::format_pieces): Handle \e.
	* unittests/format_pieces-selftests.c: New.

1 files changed, 8 insertions, 5 deletions

diff --git a/gdb/common/format.h b/gdb/common/format.h
index 1745517..7b053f1 100644
--- a/gdb/common/format.h
+++ b/gdb/common/format.h
@@ -20,6 +20,8 @@
 #ifndef COMMON_FORMAT_H
 #define COMMON_FORMAT_H
 
+#include "common/gdb_string_view.h"
+
 #if defined(__MINGW32__) && !defined(PRINTF_HAS_LONG_LONG)
 # define USE_PRINTF_I64 1
 # define PRINTF_HAS_LONG_LONG
@@ -54,6 +56,12 @@ struct format_piece
   {
   }
 
+  bool operator== (const format_piece &other) const
+  {
+    return (this->argclass == other.argclass
+	    && gdb::string_view (this->string) == other.string);
+  }
+
   const char *string;
   enum argclass argclass;
 };
@@ -67,11 +75,6 @@ public:
 
   DISABLE_COPY_AND_ASSIGN (format_pieces);
 
-  format_piece &operator[] (size_t index)
-  {
-    return m_pieces[index];
-  }
-
   typedef std::vector<format_piece>::iterator iterator;
 
   iterator begin ()