Merge pull request #35 from cksource/t/34

Checking out to specific commit (hash) works as it should.

lib/commands/update.js

@@ -55,8 +55,7 @@ module.exports = {
 					if ( stdout ) {
 						throw new Error( `Package "${ data.packageName }" has uncommitted changes. Aborted.` );
 					}
-				} )
-				.then( () => {
+
 					return execCommand.execute( getExecData( `git fetch` ) );
 				} )
 				.then( ( response ) => {
@@ -67,6 +66,20 @@ module.exports = {
 				} )
 				.then( ( response ) => {
 					log.concat( response.logs );
+				} )
+				.then( () => {
+					return execCommand.execute( getExecData( 'git branch' ) );
+				} )
+				.then( ( response ) => {
+					const stdout = response.logs.info.join( '\n' ).trim();
+					const isOnBranchRegexp = /HEAD detached at [\w\d]+/;
+
+					// If on a detached commit, mgit must not pull the changes.
+					if ( isOnBranchRegexp.test( stdout ) ) {
+						log.info( `Package "${ data.packageName }" is on a detached commit.` );
+
+						return resolve( { logs: log.all() } );
+					}
 
 					return execCommand.execute( getExecData( `git pull origin ${ data.repository.branch }` ) );
 				} )