Update copy used in alert message when deleting branches or tags.

Reference: #2993 

Before:

![Screen_Shot_2015-11-19_at_4.10.34_PM](/uploads/5cdf7e8c5d4c6bd9e18584b118420c8c/Screen_Shot_2015-11-19_at_4.10.34_PM.png)

![Screen_Shot_2015-11-19_at_4.14.19_PM](/uploads/80513b96492122860733462efa8c8015/Screen_Shot_2015-11-19_at_4.14.19_PM.png)

After:

![Screen_Shot_2015-11-19_at_4.59.56_PM](/uploads/80367f2acea0d22d36a5297bdc29163e/Screen_Shot_2015-11-19_at_4.59.56_PM.png)

![Screen_Shot_2015-11-19_at_4.59.30_PM](/uploads/134cf02d1fb3feff2159b5659d149d54/Screen_Shot_2015-11-19_at_4.59.30_PM.png)

See merge request !1846