Fix grammar for message when only one commit is abandoned after Git import

[emesterhazy]

Before:

• "Abandoned 1 commits that are no longer reachable."

After:

• "Abandoned 1 commit that is no longer reachable."

[yuja]

Before:
* "Abandoned 1 commits that are no longer reachable."
After:
* "Abandoned 1 commit that is no longer reachable."

I'm not strongly against this, but I personally prefer not having singular/plural conditionals everywhere. I don't think it's worth the added complexity.

[joyously]

I think it's best handled within a translation framework.

[emesterhazy]

RE: complexity, I agree. It's not ideal. But the current message reads very oddly when only one commit is abandoned. This is definitely just polish, so we could ignore it for now and implement something a little less ad-hoc.

RE: A translation framework. I'm not very familiar with how that might work, but I guess that's probably true. Maybe https://github.com/unicode-org/icu4x if the goal is to actually output messages in different languages.

[zummenix]

This message actually also bothered me initially. One alternative I can think of: Abandoned 1 commit(s) that are no longer reachable.

[martinvonz]

FWIW, I agree with Yuya. I don't even think of "1 commits" as sounding weird (and my native language does have plurals, in case you're wondering). But it does seem to bother many others because we've seen several patches like this. I think there are many more places to update if you want to do it consistently :)

[emesterhazy]

Thanks everyone for the feedback. I think that we should add this polish, but it should probably be done in a less ad-hoc way per the suggestion from @joyously. I'll drop this PR for now and come back with something better in the future.