installer: add option for external GnuPG

This pull request addresses git-for-windows/git/issues/3997 by adding an
additional detection mechanism for externally supplied GnuPG binaries to a new
choice page in the installer and allows the user to "skip" the installation of
the bundled GPG related binaries.

Signed-off-by: Nikolas Grottendieck <git@nikolasgrottendieck.com>

installer/install.iss

@@ -365,6 +365,10 @@ const
     GC_OpenSSL        = 1;
     GC_WinSSL         = 2;
 
+    // Git GPG options.
+    GG_GPG            = 1;
+    GG_ExternalGPG    = 2;
+
     // Git line ending conversion options.
     GC_LFOnly         = 1;
     GC_CRLFAlways     = 2;
@@ -476,6 +480,10 @@ var
     CurlVariantPage:TWizardPage;
     RdbCurlVariant:array[GC_OpenSSL..GC_WinSSL] of TRadioButton;
 
+    // Wizard page and variables for the GPG options.
+    GPGChoicePage:TWizardPage;
+    RdbGPG:array[GG_GPG..GG_ExternalGPG] of TRadioButton;
+
     // Wizard page and variables for the line ending conversion options.
     CRLFPage:TWizardPage;
     RdbCRLF:array[GC_LFOnly..GC_CRLFCommitAsIs] of TRadioButton;
@@ -2211,6 +2219,34 @@ begin
         RdbCurlVariant[GC_OpenSSL].Checked:=True;
     end;
 
+    (*
+     * Create a custom page for using self-supplied GPG instead of bundled GPG
+     * if an GPG binary is found on the PATH.
+     *)
+
+    if (FileSearch('gpg.exe', GetEnv('PATH')) <> '') then begin
+        GPGChoicePage:=CreatePage(PrevPageID,'Choosing the GPG executable','Which GnuPG program would you like Git to use?',TabOrder,Top,Left);
+
+        // 1st choice
+        RdbGPG[GG_GPG]:=CreateRadioButton(GPGChoicePage,'Use bundled GPG','This uses gpg.exe that comes with Git.',TabOrder,Top,Left);
+
+        // 2nd choice
+        RdbGPG[GG_ExternalGPG]:=CreateRadioButton(GPGChoicePage,'Use external GPG',
+            '<RED>NEW!</RED> This uses an external gpg.exe. Git will not install its own GnuPG'+#13+
+            '(and related) binaries but use them as found on the PATH.',
+            TabOrder,Top,Left);
+
+        // Restore the setting chosen during a previous install.
+        case ReplayChoice('GPG Option','GPG') of
+            'GPG': RdbGPG[GG_GPG].Checked:=True;
+            'ExternalGPG': RdbGPG[GG_ExternalGPG].Checked:=True;
+        else
+            RdbGPG[GG_GPG].Checked:=True;
+        end;
+    end else begin
+        GPGChoicePage:=NIL;
+    end;
+
     (*
      * Create a custom page for the core.autocrlf setting.
      *)
@@ -3385,6 +3421,14 @@ begin
     end;
 #endif
 
+#ifdef DELETE_GPG_FILES
+    if (GPGChoicePage<>NIL) and (RdbGPG[GG_ExternalGPG].Checked) then begin
+        WizardForm.StatusLabel.Caption:='Removing bundled Git GPG binaries';
+        if not DeleteGPGFiles() then
+            LogError('Failed to remove GPG file(s)');
+    end;
+#endif
+
     {
         Set the default Git editor
     }

installer/release.sh

@@ -296,6 +296,18 @@ openssh_deletes="$(comm -12 sorted-file-list.txt sorted-openssh-file-list.txt |
 inno_defines="$inno_defines$LF[Code]${LF}function DeleteOpenSSHFiles():Boolean;${LF}var$LF    AppDir:String;${LF}begin$LF    AppDir:=ExpandConstant('{app}');$LF    Result:=True;"
 inno_defines="$inno_defines$LF$openssh_deletes${LF}end;$LF#define DELETE_OPENSSH_FILES 1"
 
+# 1. Collect all GPG related files from $LIST and pacman, sort each and then return the overlap
+# 2. Convert paths to Windows filesystem compatible ones and construct the function body for the DeleteGPGFiles function; one DeleteFile operation per file found
+# 3. Construct DeleteGPGFiles function signature to be used in install.iss
+# 4. Assemble function body and compile flag to be used as guard in install.iss
+echo "$LIST" | sort >sorted-file-list.txt
+pacman -Ql gnupg 2>pacman.stderr | sed -n 's|^gnupg /\(.*[^/]\)$|\1|p' | sort >sorted-gnupg-file-list.txt
+grep -v 'database file for .* does not exist' <pacman.stderr >&2
+gpg_deletes="$(comm -12 sorted-file-list.txt sorted-gnupg-file-list.txt |
+	sed -e 'y/\//\\/' -e "s|.*|    if not DeleteFile(AppDir+'\\\\&') then\n        Result:=False;|")"
+inno_defines="$inno_defines$LF[Code]${LF}function DeleteGPGFiles():Boolean;${LF}var$LF    AppDir:String;${LF}begin$LF    AppDir:=ExpandConstant('{app}');$LF    Result:=True;"
+inno_defines="$inno_defines$LF$gpg_deletes${LF}end;$LF#define DELETE_GPG_FILES 1"
+
 test -z "$LIST" ||
 echo "$LIST" |
 sed -e 's|/|\\|g' \