Add some more checking/validation methods to PBGitRepository.

- Add a shaExists method that takes string with a SHA and either returns the full
  SHA if it exists or nil if not.
- Add a completeRefForString: method which completes partial refs like "xyz/master"
  to "refs/remotes/xyz/master" for example.
- Add a checkRefFormatForBranch: method which does the same as checkRefFormat 
  except it also works with partial branch refs like "xyz/master".
- Add headRefForSHA skeleton to encapsulate the shell-script-fu from populateList
  in PBGitSidebarController that deals with getting the head ref for a SHA.

PBGitRepository.h

@@ -103,7 +103,11 @@ static NSString * PBStringFromBranchFilterType(PBGitXBranchFilterType type) {
 - (BOOL) isSHAOnHeadBranch:(NSString *)testSHA;
 - (BOOL) isRefOnHeadBranch:(PBGitRef *)testRef;
 - (BOOL) checkRefFormat:(NSString *)refName;
+- (BOOL) checkRefFormatForBranch:(NSString *)shaOrRefName;
+- (PBGitRef *) completeRefForString:(NSString *)partialRefString;
 - (BOOL) refExists:(PBGitRef *)ref;
+// checks to see if the ref or sha exists - returns nil if not found and the complete sha if found
+- (NSString *) shaExists:(NSString *)sha;
 
 - (NSArray *) remotes;
 - (BOOL) hasRemotes;

PBGitRepository.m

@@ -311,6 +311,13 @@ static NSString * repositoryBasePath = nil;
 	return [self shaForRef:[[self headRef] ref]];
 }
 
+- (NSString *) headRefForSHA:(NSString *)sha
+{
+	// TODO: move code over
+    // from PBGitSidebarController populateList
+    // that deals with getting the head refs path
+}
+
 - (PBGitCommit *) headCommit
 {
 	return [self commitForSHA:[self headSHA]];
@@ -417,6 +424,32 @@ static NSString * repositoryBasePath = nil;
 	return YES;
 }
 
+- (BOOL) checkRefFormatForBranch:(NSString *)shaOrRefName
+{
+	int retValue = 1;
+	[self outputInWorkdirForArguments:[NSArray arrayWithObjects:@"check-ref-format", @"--branch", shaOrRefName, nil] retValue:&retValue];
+	if (retValue)
+		return NO;
+	return YES;
+}
+
+- (PBGitRef *) completeRefForString:(NSString *)partialRefString {
+    if (![self checkRefFormat:partialRefString] &&
+        ![self checkRefFormatForBranch:partialRefString]) {
+        return nil;
+    }
+    NSArray * prefixes = [NSArray arrayWithObjects:kGitXBranchRefPrefix, kGitXRemoteRefPrefix, kGitXTagRefPrefix, nil];
+    PBGitRef * completeRef = nil;
+    for (NSString * prefix in prefixes) {
+        PBGitRef * tryref = [PBGitRef refFromString:[NSString stringWithFormat:@"%@%@", prefix, partialRefString]];
+        if ([self refExists:tryref]) {
+            completeRef = tryref;
+            break;
+        }
+    }
+    return completeRef;
+}
+
 - (BOOL) refExists:(PBGitRef *)ref
 {
 	int retValue = 1;
@@ -425,6 +458,15 @@ static NSString * repositoryBasePath = nil;
         return NO;
     return YES;
 }
+
+- (NSString *) shaExists:(NSString *)sha
+{
+	int retValue = 1;
+    NSString *output = [self outputInWorkdirForArguments:[NSArray arrayWithObjects:@"rev-parse", sha, nil] retValue:&retValue];
+    if (retValue || [output isEqualToString:@""])
+        return nil;
+    return output;
+}
 		
 // Returns either this object, or an existing, equal object
 - (PBGitRevSpecifier*) addBranch:(PBGitRevSpecifier*)branch