Add new field for push-options

To fix a segfault at [1], adjust to upstream development[2,3] that
changed the shape of this struct.

This new field is added in the same style as `custom_headers`.

[1]: libgit2.git:39669956fb510fb7b13289f6ce959884969dbebd:src/libgit2/remote.c:2987
[2]: libgit2.git:ecc6f2fb8399d84e5b2bf043376dfc51f43f3e90
[3]: libgit2.git:39669956fb510fb7b13289f6ce959884969dbebd

Author: vermiculus

libgit2-sys/lib.rs

@@ -981,6 +981,7 @@ pub struct git_push_options {
     pub proxy_opts: git_proxy_options,
     pub follow_redirects: git_remote_redirect_t,
     pub custom_headers: git_strarray,
+    pub remote_push_options: git_strarray,
 }
 
 pub type git_tag_foreach_cb =

src/remote.rs

@@ -58,6 +58,8 @@ pub struct PushOptions<'cb> {
     follow_redirects: RemoteRedirect,
     custom_headers: Vec<CString>,
     custom_headers_ptrs: Vec<*const c_char>,
+    remote_push_options: Vec<CString>,
+    remote_push_options_ptrs: Vec<*const c_char>,
 }
 
 /// Holds callbacks for a connection to a `Remote`. Disconnects when dropped
@@ -628,6 +630,8 @@ impl<'cb> PushOptions<'cb> {
             follow_redirects: RemoteRedirect::Initial,
             custom_headers: Vec::new(),
             custom_headers_ptrs: Vec::new(),
+            remote_push_options: Vec::new(),
+            remote_push_options_ptrs: Vec::new(),
         }
     }
 
@@ -673,6 +677,16 @@ impl<'cb> PushOptions<'cb> {
         self.custom_headers_ptrs = self.custom_headers.iter().map(|s| s.as_ptr()).collect();
         self
     }
+
+    /// Set extra headers for this push operation.
+    pub fn remote_push_options(&mut self, remote_push_options: &[&str]) -> &mut Self {
+        self.remote_push_options = remote_push_options
+            .iter()
+            .map(|&s| CString::new(s).unwrap())
+            .collect();
+        self.remote_push_options_ptrs = self.remote_push_options.iter().map(|s| s.as_ptr()).collect();
+        self
+    }
 }
 
 impl<'cb> Binding for PushOptions<'cb> {
@@ -700,6 +714,10 @@ impl<'cb> Binding for PushOptions<'cb> {
                 count: self.custom_headers_ptrs.len(),
                 strings: self.custom_headers_ptrs.as_ptr() as *mut _,
             },
+            remote_push_options: git_strarray {
+                count: self.remote_push_options_ptrs.len(),
+                strings: self.remote_push_options_ptrs.as_ptr() as *mut _,
+            },
         }
     }
 }