find-releases: handle untagged merges
find-releases: handle untagged merges

At the moment, this doesn't work too well for something that was just merged to
45.

[(b912ace...)]scottmg@scottmg:/work/cr/src$ git find-releases 55b4c95889d6a72e52bc72702580a62f04f35777
commit 55b4c95889d6a72e52bc72702580a62f04f35777 was:
  initially in 47.0.2501.0
  merged to undefined (as de54b58d404a0cdf01639ae7260ef10bbb843ed9)
  merged to 46.0.2490.19 (as b4ba1071baaabc92cb5a2ca89e238c478eca0075)

("undefined" being the problem). This is because:

[(b912ace...)]scottmg@scottmg:/work/cr/src$ git name-rev --tags de54b58d404a0cdf01639ae7260ef10bbb843ed9
de54b58d404a0cdf01639ae7260ef10bbb843ed9 undefined

But,

[(b912ace...)]scottmg@scottmg:/work/cr/src$ cat .git/refs/remotes/branch-heads/2454
de54b58d404a0cdf01639ae7260ef10bbb843ed9

So, if name-rev --tags says it's not yet tagged, rather than just printing
'undefined', at least say which branch it was merged to. As in:

[(b912ace...)]scottmg@scottmg:/work/cr/src$ git find-releases 55b4c95889d6a72e52bc72702580a62f04f35777
commit 55b4c95889d6a72e52bc72702580a62f04f35777 was:
  initially in 47.0.2501.0
  merged to branch-heads/2454 [untagged] (as de54b58d404a0cdf01639ae7260ef10bbb843ed9)
  merged to 46.0.2490.19 (as b4ba1071baaabc92cb5a2ca89e238c478eca0075)


R=dpranke@chromium.org,primiano@chromium.org

Review URL: https://codereview.chromium.org/1325233005

git-svn-id: fd409f4bdeea2bb50a5d34bb4d4bfc2046a5a3dd@296621 0039d316-1c4b-4281-b951-d872f2087c98

mlufei/depot_tools,duanwujie/depot_tools,mlufei/depot_tools,CoherentLabs/depot_tools,aleonliao/depot_tools,kaiix/depot_tools,duongbaoduy/gtools,disigma/depot_tools,hsharsha/depot_tools,Midrya/chromium,azureplus/chromium_depot_tools,Midrya/chromium,CoherentLabs/depot_tools,azureplus/chromium_depot_tools,duongbaoduy/gtools,aleonliao/depot_tools,disigma/depot_tools,duanwujie/depot_tools,hsharsha/depot_tools,aleonliao/depot_tools,primiano/depot_tools,kaiix/depot_tools,duongbaoduy/gtools,hsharsha/depot_tools,azureplus/chromium_depot_tools,primiano/depot_tools,kaiix/depot_tools,mlufei/depot_tools,disigma/depot_tools,primiano/depot_tools,Midrya/chromium,duanwujie/depot_tools