MergeException: New changes were pushed to master in PROJECT/repo while the merge was being performed

2013-11-21 22:02:58,437 DEBUG [http-bio-7990-exec-1] aefimov 1322x3030x0 kcu9ej 127.0.0.1 "POST /rest/api/latest/projects/PROJECT/repos/repo/pull-requests/6
349/merge HTTP/1.0" c.a.s.r.e.ServiceExceptionMapper Mapping ServiceException to REST response 409
com.atlassian.stash.scm.MergeException: New changes were pushed to master in PROJECT/repo while the merge was being performed. Please retry the merge.
        at com.atlassian.stash.internal.scm.git.merge.MergeCommand.newMergeException(MergeCommand.java:138) ~[na:na]
        at com.atlassian.stash.internal.scm.git.merge.MergeCommand.doWithWorkTree(MergeCommand.java:116) ~[na:na]
        at com.atlassian.stash.internal.scm.git.merge.MergeCommand.doWithWorkTree(MergeCommand.java:29) ~[na:na]
        at com.atlassian.stash.internal.scm.git.porcelain.AbstractWorkTreeCommand.call(AbstractWorkTreeCommand.java:38) ~[na:na]
        at com.atlassian.stash.internal.pull.PullRequestServiceImpl$MergeAndUpdatePullRequestOperation.perform(PullRequestServiceImpl.java:1721) ~[stash-service
-impl-2.9.1.jar:na]
        at com.atlassian.stash.internal.pull.PullRequestServiceImpl$MergeAndUpdatePullRequestOperation.perform(PullRequestServiceImpl.java:1693) ~[stash-service
-impl-2.9.1.jar:na]
        at com.atlassian.stash.internal.concurrent.DefaultLockService.doWithLock(DefaultLockService.java:77) ~[stash-service-impl-2.9.1.jar:na]
        at com.atlassian.stash.internal.concurrent.DefaultLockService.doWithLock(DefaultLockService.java:64) ~[stash-service-impl-2.9.1.jar:na]
        at com.sun.proxy.$Proxy162.doWithLock(Unknown Source) ~[na:na]
        at com.atlassian.stash.internal.pull.PullRequestServiceImpl.merge(PullRequestServiceImpl.java:623) ~[stash-service-impl-2.9.1.jar:na]
        at com.sun.proxy.$Proxy171.merge(Unknown Source) ~[na:na]
        at com.sun.proxy.$Proxy245.merge(Unknown Source) ~[na:na]
        at com.sun.proxy.$Proxy516.merge(Unknown Source) ~[na:na]
        at com.atlassian.stash.internal.rest.pull.PullRequestResource.mergePullRequest(PullRequestResource.java:423) ~[stash-rest-2.9.1.jar:na]
        at com.atlassian.applinks.core.rest.context.ContextFilter.doFilter(ContextFilter.java:25) [applinks-plugin-4.0.5_1384476184000.jar:na]
        at com.atlassian.stash.internal.spring.security.StashAuthenticationFilter.doFilter(StashAuthenticationFilter.java:90) [StashAuthenticationFilter.class:n
a]
        at com.atlassian.stash.internal.web.auth.BeforeLoginPluginAuthenticationFilter.doInsideSpringSecurityChain(BeforeLoginPluginAuthenticationFilter.java:11
1) [BeforeLoginPluginAuthenticationFilter.class:na]
        at com.atlassian.stash.internal.web.auth.BeforeLoginPluginAuthenticationFilter.doFilter(BeforeLoginPluginAuthenticationFilter.java:77) [BeforeLoginPlugi
nAuthenticationFilter.class:na]
        at com.atlassian.security.auth.trustedapps.filter.TrustedApplicationsFilter.doFilter(TrustedApplicationsFilter.java:100) [atlassian-trusted-apps-core-3.
0.2.jar:na]
        at com.atlassian.oauth.serviceprovider.internal.servlet.OAuthFilter.doFilter(OAuthFilter.java:61) [atlassian-oauth-service-provider-plugin-1.9.0-m3_1384
476184000.jar:na]
        at com.atlassian.stash.internal.web.auth.BeforeLoginPluginAuthenticationFilter.doBeforeBeforeLoginFilters(BeforeLoginPluginAuthenticationFilter.java:89)
 [BeforeLoginPluginAuthenticationFilter.class:na]
        at com.atlassian.stash.internal.web.auth.BeforeLoginPluginAuthenticationFilter.doFilter(BeforeLoginPluginAuthenticationFilter.java:75) [BeforeLoginPlugi
nAuthenticationFilter.class:na]
        at com.atlassian.stash.internal.request.DefaultRequestManager.doAsRequest(DefaultRequestManager.java:72) [stash-service-impl-2.9.1.jar:na]
        at java.lang.Thread.run(Thread.java:662) [na:1.6.0_45]
        ... 243 frames trimmed
Caused by: com.atlassian.stash.scm.git.push.NonFastForwardUpdateRejectedException: The merge could not be completed because the target ref was updated mid-merge
.
        at com.atlassian.stash.internal.scm.git.command.merge.UpdateRefCommandExitHandler.evaluateStdErr(UpdateRefCommandExitHandler.java:27) ~[na:na]
        at com.atlassian.stash.scm.git.common.command.GitCommandExitHandler.onError(GitCommandExitHandler.java:159) ~[na:na]
        at com.atlassian.stash.scm.DefaultCommandExitHandler.onExit(DefaultCommandExitHandler.java:35) ~[stash-spi-2.9.1.jar:na]
        at com.atlassian.stash.scm.BaseCommand.callExitHandler(BaseCommand.java:141) ~[stash-spi-2.9.1.jar:na]
        at com.atlassian.stash.scm.BaseCommand$CommandFuture.internalGet(BaseCommand.java:257) ~[stash-spi-2.9.1.jar:na]
        at com.atlassian.stash.scm.BaseCommand$CommandFuture.get(BaseCommand.java:224) ~[stash-spi-2.9.1.jar:na]
        at com.atlassian.stash.scm.BaseCommand.call(BaseCommand.java:78) ~[stash-spi-2.9.1.jar:na]
        at com.atlassian.stash.internal.scm.git.porcelain.AbstractTimedRequest.configureAndCall(AbstractTimedRequest.java:30) ~[na:na]