Change Amend Commit menu item to use an action instead of bindings.

Bindings can make the menu item behavior act erratically because the binding state is shared across the app, not per-document.
This also adds a checkmark to the menu item when amending is active.

Steps to reproduce a problem like this:
- Open a repository
- Select Amend Commit to start amending
- Close the repository
- Open the same repository
- Select Amend Commit to start amending again
- This time the menu item sets amend to NO instead of YES, so nothing happens the first time you select the menu item

Author: ksuther

Classes/Controllers/PBGitCommitController.m

@@ -253,6 +253,11 @@ - (IBAction) forceCommit:(id) sender
     [self commitWithVerification:NO];
 }
 
+- (IBAction)toggleAmendCommit:(id)sender
+{
+	[[[self repository] index] setAmend:![[[self repository] index] isAmend]];
+}
+
 - (NSArray <PBChangedFile *> *)selectedFilesForSender:(id)sender
 {
 	NSParameterAssert(sender != nil);
@@ -692,6 +697,9 @@ - (BOOL)validateMenuItem:(NSMenuItem *)menuItem
 		}
 		return active;
 	}
+	else if (menuItem.action == @selector(toggleAmendCommit:)) {
+		menuItem.state = [[[self repository] index] isAmend] ? NSOnState : NSOffState;
+	}
 
 	return menuItem.enabled;
 }

Resources/English.lproj/MainMenu.xib

@@ -528,7 +528,7 @@ DQ
                             <menuItem title="Amend Commit" keyEquivalent="a" id="W5p-JV-xeS">
                                 <modifierMask key="keyEquivalentModifierMask" option="YES" command="YES"/>
                                 <connections>
-                                    <binding destination="oqX-ct-wMx" name="value" keyPath="currentDocument.windowController.repository.index.amend" id="qcF-1I-xfx"/>
+                                    <action selector="toggleAmendCommit:" target="-1" id="sQs-hs-7cR"/>
                                 </connections>
                             </menuItem>
                         </items>