Show friendly errors when ref is not found

[agis]

Before:

$ bundle install
Fetching git://github.com/agis-/some_gem.git
fatal: ambiguous argument 'FOO': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git  [...] -- [...]'
Git error: command `git rev-parse WRONG` in directory /cache/path
has failed.
Ref 'foo' was not found. Perhaps you mispelled it?
If this error persists you could try removing the cache directory
# ...

After:

$ bundle install
Fetching git://github.com/agis-/some_gem.git
fatal: Needed a single revision
Git error: command `git rev-parse --verify WRONG` in directory /cache/path
has failed.
Ref 'foo' was not found. Perhaps you mispelled it?
If this error persists you could try removing the cache directory
# ...

Fixes #3879.

[indirect]

Looks good to me. @homu r+

[homu]

📌 Commit b8da6a9 has been approved by indirect

[homu]

⌛ Testing commit b8da6a9 with merge fa9c4e2...

[homu]

☀️ Test successful - status