According to Andrey Dernov from JetBrains Support:
You use abbreviation added in keymap for quick search an action when "Search everywhere" (double Shift).
After adding "foobar" abbreviation to Git annotation command.
I can find the abbreviation and invoke the command with double Shift.
Unfortunately, the functionality does not support partial entry.
I personally find Shift-⌘-A easier to use in that case, because the results are limited to commands only.
PS: JetBrains support rocks!
Update: There was a follow-up from JetBrains:
The feature was recently added in IDEA, we haven't had time to update the documentation yet. We will do it in near future.