feat: check if password file exists before running pass

[ktetzlaff]

Currently, pass-git-helper doesn't check if a password file exists before running pass. This leads to authentication failures when the configured password store entry (aka target) points to a directory instead of a file.

Example:

$ tree -F .password-store
.password-store/
└── git/
    ├── github.com.gpg
    └── gitlab.com/
        ├── user1.gpg
        └── user2.gpg

If target is git/gitlab.com, pass-git-helper will get the following output from pass git/gitlab/com:

git/gitlab.com
├── user1
└── user2

and will then use the first line (git/gitlab/com) as password.

This commit introduces the following changes:

1. To fix the problem described above, pass-git-helper first determines the pass password store directory as:

   • the value of password_store_dir if defined in git_pass_mapping.ini (and non-empty)
   • the value of the environment variable PASSWORD_STORE_DIR if defined (and non-empty)
   • the default: ~/.password-store

   (the latter two correspond to the implementation of pass).
   pass-git-helper will then check if an actual <target>.gpg is present in
   the selected directory (to be clear: only one of the three alternative
   directories gets checked).

   The new checks implemented in pass-git-helper detect two different error
   cases:

   1. the scenario described above where target itself is a directory
   2. a (rather obscure) scenario where <target>.gpg is a directory

   In both cases, pass-git-helper will now exit with an error after presenting
   the user with a (hopefully) useful error message.

2. The value of password_store_dir in the ini file may now contain a leading ~ (tilde) which will get replaced by the users HOME (or, on Windows, USERPROFILE) directory. The change is documented in README.md.

3. New tests for 1. and 2.

Fixes #371

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.66%. Comparing base (0489e36) to head (b50ca3b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #372      +/-   ##
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+ Coverage   91.37%   91.66%   +0.28%     
==========================================
  Files           1        1              
  Lines         174      180       +6     
  Branches       23       24       +1     
==========================================
+ Hits          159      165       +6     
  Misses         10       10              
  Partials        5        5              

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[languitar]

Thanks for the great contribution. I have added a few annotations for mostly style issues to match the current implementation.

Additionally, as the changelog for the release is automatically built from the commit messages, can you split this PR up into two commits. One for the better error message in case an extractor name is off and a second commit for the actual feature of this PR?

[ktetzlaff]

... split this PR up into two commits. One for the better error message in case an extractor name is off

The username extractor changes are now in in the new PR #375.

[ktetzlaff]

@languitar I think I resolved all open issues. Please check.

[languitar]

Thanks again for the nice contribution and for updating the PR after the review.