Bravely have read_blocked return after first read

In commit fd6d814ea4, read_blocked was changed to read until EOF
or the full amount requested is returned. Switch this to returning
as soon as any data is available, which was the behavior prior to
fd6d814ea4.

This will allow builtin_string to output data in a "streaming"
fashion instead of needing to read a large block up-front.

src/common.cpp

@@ -474,23 +474,11 @@ void fish_setlocale() {
 }
 
 long read_blocked(int fd, void *buf, size_t count) {
-    long bytes_read = 0;
-
-    while (count) {
-        ssize_t res = read(fd, static_cast<char *>(buf) + bytes_read, count);
-        if (res == 0) {
-            break;
-        } else if (res == -1) {
-            if (errno == EINTR) continue;
-            if (errno == EAGAIN) return bytes_read ? bytes_read : -1;
-            return -1;
-        } else {
-            bytes_read += res;
-            count -= res;
-        }
-    }
-
-    return bytes_read;
+    ssize_t res;
+    do {
+        res = read(fd, buf, count);
+    } while (res < 0 && errno == EINTR);
+    return res;
 }
 
 /// Loop a write request while failure is non-critical. Return -1 and set errno in case of critical