adb: fix escape_arg for multiple quotes

escape_arg reuses the same index for the source (s) and the destination
(result), so it breaks on strings containing more than one quote, e.g:

 * a'b'c    ==> 'a'\''b'c'
 * a'bcde'f ==> 'a'\''b'\'cde'f'

Also make the function more efficient by doing fewer string copies. This
code is based on the android::base::Split code.

Use EXPECT_EQ because the tests can keep going if one fails.

Bug: none
Test: adb_test --gtest_filter=adb_utils.escape_arg
Change-Id: I6ca6e905fa53cc61b9a87276cb7116a5df7e8017
diff --git a/adb_utils.cpp b/adb_utils.cpp
index b236fb3..0c3327f 100644
--- a/adb_utils.cpp
+++ b/adb_utils.cpp
@@ -79,22 +79,24 @@
 }
 
 std::string escape_arg(const std::string& s) {
-  std::string result = s;
-
   // Escape any ' in the string (before we single-quote the whole thing).
   // The correct way to do this for the shell is to replace ' with '\'' --- that is,
   // close the existing single-quoted string, escape a single single-quote, and start
   // a new single-quoted string. Like the C preprocessor, the shell will concatenate
   // these pieces into one string.
-  for (size_t i = 0; i < s.size(); ++i) {
-    if (s[i] == '\'') {
-      result.insert(i, "'\\'");
-      i += 2;
-    }
+
+  std::string result;
+  result.push_back('\'');
+
+  size_t base = 0;
+  while (true) {
+    size_t found = s.find('\'', base);
+    result.append(s, base, found - base);
+    if (found == s.npos) break;
+    result.append("'\\''");
+    base = found + 1;
   }
 
-  // Prefix and suffix the whole string with '.
-  result.insert(result.begin(), '\'');
   result.push_back('\'');
   return result;
 }