Fix Windows command line quoting of backslash runs: a trailing backslash, or
backslashes preceding a double quote, were not doubled, so CommandLineToArgvW
in the child parsed them as escapes and mangled the argument list.

Upstream PR: https://github.com/sheredom/subprocess.h/pull/101
Applied locally by scripts/sync_vendor.py until it is merged upstream.

(the test/ changes from the PR are omitted, we only vendor subprocess.h)

diff --git a/subprocess.h b/subprocess.h
index 5e80902..b06ad4d 100644
--- a/subprocess.h
+++ b/subprocess.h
@@ -653,6 +653,7 @@ int subprocess_create_ex(const char *const commandLine[], int options,
   int wide_len;
   int i, j;
   int need_quoting;
+  subprocess_size_t bs_run;
   unsigned long flags = 0;
   unsigned long last_error = 0;
   int result = subprocess_error_unknown;
@@ -906,25 +907,29 @@ int subprocess_create_ex(const char *const commandLine[], int options,
     len++;
 
     // Quote the argument if it has a space in it
-    if (strpbrk(commandLine[i], "\t\v ") != SUBPROCESS_NULL ||
-        commandLine[i][0] == SUBPROCESS_NULL)
+    need_quoting = strpbrk(commandLine[i], "\t\v ") != SUBPROCESS_NULL ||
+                   commandLine[i][0] == SUBPROCESS_NULL;
+    if (need_quoting)
       len += 2;
 
+    bs_run = 0;
     for (j = 0; '\0' != commandLine[i][j]; j++) {
-      switch (commandLine[i][j]) {
-      default:
-        break;
-      case '\\':
-        if (commandLine[i][j + 1] == '"') {
-          len++;
-        }
+      len++;
 
-        break;
-      case '"':
-        len++;
-        break;
+      if ('\\' == commandLine[i][j]) {
+        bs_run++;
+      } else {
+        if ('"' == commandLine[i][j]) {
+          // Duplicate the preceding run and escape the quote.
+          len += bs_run + 1;
+        }
+        bs_run = 0;
       }
-      len++;
+    }
+
+    if (need_quoting) {
+      // Duplicate trailing slashes before the generated closing quote.
+      len += bs_run;
     }
   }
 
@@ -949,22 +954,29 @@ int subprocess_create_ex(const char *const commandLine[], int options,
       commandLineCombined[len++] = '"';
     }
 
-    for (j = 0; '\0' != commandLine[i][j]; j++) {
-      switch (commandLine[i][j]) {
-      default:
-        break;
-      case '\\':
-        if (commandLine[i][j + 1] == '"') {
-          commandLineCombined[len++] = '\\';
-        }
+    for (j = 0; '\0' != commandLine[i][j];) {
+      bs_run = 0;
+      while ('\\' == commandLine[i][j]) {
+        bs_run++;
+        j++;
+      }
+
+      if ('"' == commandLine[i][j]) {
+        // 2n + 1 slashes preserve n slashes and escape the quote.
+        bs_run = (bs_run * 2) + 1;
+      } else if ('\0' == commandLine[i][j] && need_quoting) {
+        // 2n slashes preserve n slashes before the closing quote.
+        bs_run *= 2;
+      }
 
-        break;
-      case '"':
+      while (bs_run > 0) {
         commandLineCombined[len++] = '\\';
-        break;
+        bs_run--;
       }
 
-      commandLineCombined[len++] = commandLine[i][j];
+      if ('\0' != commandLine[i][j]) {
+        commandLineCombined[len++] = commandLine[i][j++];
+      }
     }
     if (need_quoting) {
       commandLineCombined[len++] = '"';