opam update stops working if an installed package's url changes from tarball to git

[mato]

As I discovered today while moving packages around in the Solo5 staging repository, if a package is installed in an opam switch, and that package's url changes from a release tarball to a Git repository, opam update will start failing with this message:

$ opam update
[ERROR] Command "/usr/bin/git diff --no-ext-diff --quiet" failed:
"/usr/bin/git diff --no-ext-diff --quiet" exited with code 129

$ cat ~/.opam/log/log-20708-6d07ae.info 
# path        ~/.opam/solo5-4.09.0/.opam-switch/sources/solo5-bindings-hvt.0.6.2
# command     /usr/bin/git diff --no-ext-diff --quiet
# env-file    ~/.opam/log/log-20708-6d07ae.env
# output-file ~/.opam/log/log-20708-6d07ae.out
$ cat ~/.opam/log/log-20708-6d07ae.out 
Not a git repository
To compare two paths outside a working tree:
usage: git diff [--no-index] <path> <path>

I had to strace opam to find the path to the above log file, it would also be useful to give a pointer to this if commands such as this fail.

[rjbou]

Thanks for the report!