win32: make posix emulation retries configurable

POSIX emulation retries should be configurable so that tests can disable
them.  In particular, maniacally threading tests may end up trying to
open locked files and need retries, which will slow continuous
integration tests significantly.

src/win32/posix.h

@@ -15,6 +15,7 @@
 #include "dir.h"
 
 extern unsigned long git_win32__createfile_sharemode;
+extern int git_win32__retries;
 
 typedef SOCKET GIT_SOCKET;
 

src/win32/posix_w32.c

@@ -37,6 +37,7 @@ typedef DWORD(WINAPI *PFGetFinalPathNameByHandleW)(HANDLE, LPWSTR, DWORD, DWORD)
 
 unsigned long git_win32__createfile_sharemode =
  FILE_SHARE_READ | FILE_SHARE_WRITE;
+int git_win32__retries = 10;
 
 GIT_INLINE(void) set_errno(void)
 {
@@ -162,7 +163,7 @@ GIT_INLINE(bool) last_error_retryable(void)
 #define do_with_retries(fn, cleanup) \
 	do {                                                             \
 		int __tries, __ret;                                          \
-		for (__tries = 0; __tries < 10; __tries++) {                 \
+		for (__tries = 0; __tries < git_win32__retries; __tries++) { \
 			if (__tries && (__ret = (cleanup)) != 0)                 \
 				return __ret;                                        \
 			if ((__ret = (fn)) != GIT_RETRY)                         \

tests/threads/diff.c

@@ -19,12 +19,20 @@ static git_repository *_repo;
 static git_tree *_a, *_b;
 static git_atomic _counts[4];
 static int _check_counts;
+static int _retries;
 
 #define THREADS 20
 
+void test_threads_diff__initialize(void)
+{
+	_retries = git_win32__retries;
+	git_win32__retries = 1;
+}
+
 void test_threads_diff__cleanup(void)
 {
 	cl_git_sandbox_cleanup();
+	git_win32__retries = _retries;
 }
 
 static void setup_trees(void)