Make git__strntol64() ~70%* faster

This change uses compiler intrinsics to detect overflows instead of
using divisions to detect potential overflow. This makes the code faster
and makes it easier to read as a bonus side-effect!

Some of the things this quickens:

* Config parsing.
* Tree parsing.
* Smart protocol negotiation.

\* Measured by running `libgit2_clar` with `-fno-optimize-sibling-calls
-fno-omit-frame-pointer` under `perf(1)`:

```shell
$ perf diff --symbols=git__strntol64 --compute=ratio \
  --percentage=absolute baseline.data perf.data
\# Event 'cycles'
\#
\# Baseline           Ratio  Shared Object
\# ........  ..............  .............
\#
     0.25%        0.321836  libgit2_clar
```

2 files changed, 34 insertions, 11 deletions

diff --git a/src/integer.h b/src/integer.h
index 067c0be1f..026a1fac0 100644
--- a/src/integer.h
+++ b/src/integer.h
@@ -77,6 +77,11 @@ GIT_INLINE(int) git__is_int(long long p)
 # define git__sub_int_overflow(out, one, two) \
     __builtin_ssub_overflow(one, two, out)
 
+# define git__add_int64_overflow(out, one, two) \
+    __builtin_add_overflow(one, two, out)
+# define git__multiply_int64_overflow(out, one, two) \
+    __builtin_mul_overflow(one, two, out)
+
 /* Use Microsoft's safe integer handling functions where available */
 #elif defined(_MSC_VER)
 
@@ -87,11 +92,17 @@ GIT_INLINE(int) git__is_int(long long p)
     (SizeTAdd(one, two, out) != S_OK)
 # define git__multiply_sizet_overflow(out, one, two) \
     (SizeTMult(one, two, out) != S_OK)
+
 #define git__add_int_overflow(out, one, two) \
     (IntAdd(one, two, out) != S_OK)
 #define git__sub_int_overflow(out, one, two) \
     (IntSub(one, two, out) != S_OK)
 
+#define git__add_int64_overflow(out, one, two) \
+    (LongLongAdd(one, two, out) != S_OK)
+#define git__multiply_int64_overflow(out, one, two) \
+    (LongLongMult(one, two, out) != S_OK)
+
 #else
 
 /**
@@ -136,6 +147,26 @@ GIT_INLINE(bool) git__sub_int_overflow(int *out, int one, int two)
 	return false;
 }
 
+GIT_INLINE(bool) git__add_int64_overflow(int64_t *out, int64_t one, int64_t two)
+{
+	if ((two > 0 && one > (INT64_MAX - two)) ||
+	    (two < 0 && one < (INT64_MIN - two)))
+		return true;
+	*out = one + two;
+	return false;
+}
+
+GIT_INLINE(bool) git__multiply_int64_overflow(int64_t *out, int64_t one, int64_t two)
+{
+	if ((one == -1 && two == INT_MIN) ||
+	    (two == -1 && one == INT_MIN) ||
+	    (one && INT64_MAX / one < two) ||
+	    (one && INT64_MIN / one > two))
+		return true;
+	*out = one * two;
+	return false;
+}
+
 #endif
 
 #endif
diff --git a/src/util.c b/src/util.c
index 87ccf32af..af825e4d2 100644
--- a/src/util.c
+++ b/src/util.c
@@ -34,8 +34,8 @@
 int git__strntol64(int64_t *result, const char *nptr, size_t nptr_len, const char **endptr, int base)
 {
 	const char *p;
-	int64_t n, nn;
-	int c, ovfl, v, neg, ndig;
+	int64_t n, nn, v;
+	int c, ovfl, neg, ndig;
 
 	p = nptr;
 	neg = 0;
@@ -110,19 +110,11 @@ int git__strntol64(int64_t *result, const char *nptr, size_t nptr_len, const cha
 		if (v >= base)
 			break;
 		v = neg ? -v : v;
-		if (n > INT64_MAX / base || n < INT64_MIN / base) {
+		if (git__multiply_int64_overflow(&nn, n, base) || git__add_int64_overflow(&n, nn, v)) {
 			ovfl = 1;
 			/* Keep on iterating until the end of this number */
 			continue;
 		}
-		nn = n * base;
-		if ((v > 0 && nn > INT64_MAX - v) ||
-		    (v < 0 && nn < INT64_MIN - v)) {
-			ovfl = 1;
-			/* Keep on iterating until the end of this number */
-			continue;
-		}
-		n = nn + v;
 	}
 
 Return: