Support ANSI escape color codes in NINJA_STATUS

Adds support for ANSI escape sequences used for colorization to the job
status indicator, and updates `ElideMiddle()` to account for the
non-obvious change in text width.

As an example, the following will now emit the job status in green:

   env NINJA_STATUS="\\033[32m[%f/%t] \\033[0m" ninja;

Invalid color escape codes are treated as plain text (as before). ANSI
escape sequences apart from `\033...m` are not supported.

Closes ninja-build#713. Closes ninja-build#912.

src/build.cc

@@ -203,7 +203,21 @@ string BuildStatus::FormatProgressStatus(
   char buf[32];
   int percent;
   for (const char* s = progress_status_format; *s != '\0'; ++s) {
-    if (*s == '%') {
+    // Support ANSI color escape codes in NINJA_STATUS
+    if (strncmp(s, "\\033[", 5) == 0) {
+        const char *end = strchr(s + 5, 'm');
+        if (end == nullptr) {
+            // Not a valid ANSI color sequence, treat as regular text
+        } else {
+            out.append("\x1B[");
+            for (const char *t = s + 5; t <= end; ++t) {
+                out.push_back(*t);
+            }
+            s = end;
+            continue;
+        }
+    }
+    else if (*s == '%') {
       ++s;
       switch (*s) {
       case '%':

src/util.cc

@@ -570,10 +570,23 @@ double GetLoadAverage() {
 }
 #endif // _WIN32
 
+// Calculate the width of a string accounting for ANSI escape codes
+size_t CalculateWidth(const string& str) {
+    const int initial_width = str.size();
+    size_t zero_width_start = str.find("\x1B[");
+    if (zero_width_start != string::npos) {
+       size_t zero_width_end = str.find("m", zero_width_start);
+       if (zero_width_end != string::npos) {
+           return initial_width - (zero_width_end - zero_width_start) - 1;
+       }
+    }
+    return initial_width;
+}
+
 string ElideMiddle(const string& str, size_t width) {
   const int kMargin = 3;  // Space for "...".
   string result = str;
-  if (result.size() + kMargin > width) {
+  if (CalculateWidth(result) + kMargin > width) {
     size_t elide_size = (width - kMargin) / 2;
     result = result.substr(0, elide_size)
       + "..."